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I did the conversion yesterday. I did not take photo's as there are just hundreds of them on the web and Youtube being done.
What I did notice is that no one ever talks about is engine break in. I'm not a 2-stroke expert by any means but I have rebuilt many 4-stroke engines.
So what I found was:
The new cylinder head and piston ( DR Racing) came with no markings what-so-ever as to how the piston was to be placed in the cylinder. No instructions other than a flier in Italian! Not much help. You had a 50/50 chance of putting the piston in upside down! Not good. I had to email TreatlandTV and ask how to install the piston. The answer was cryptic to say the least. Not much help. I did my best guess and must have done it right.
All the gaskets and seals were replaced. I did have to separate the engine half's to replace the seals that were dry rot.( There is a well done Youtube Vid of a NU50 being converted that is over an hour long. Lots off good ideas to try.)
The cylinder does not fit in between the Honda Express foot peg brackets. You have to cut off 1/4" on both side of the top two cooling fins of the cylinder. You also have to cut 75% of one cooling fin off in order to get to the Oil Fill Plug for the transmission. One of the online service manuals show you can fill the transmission with the bike on its right side and the left transmission cover off. Good luck with that. The vent and rear bearing ( no pressure on the drive shaft from the left transmission cover while off) will leak oil like a sieve.I wont try that again.
To brake in the new rings and cylinder I purposely ran a rich pre-mix fuel/oil mix. Lots of smoke but it did not foul the plug at all. I followed the instructions from a dirt bike site on how to break in the new parts.
I asked around and found that on my 1978 NC50 the oil pump is set to a ratio of 25/1. Far richer than needed for the conversion kit larger bore. I ran the engine today for a few miles varying the RPMs and staying off max throttle.
The jet I used was a #69 in my Delorto SHA 15.15 and it was way too rich for my 7,000 ASL altitude. The engine bogged down badly. I put in a #61 and the engine really came to life. Very smooth idle and good throttle response from the stand still.
I re-torques the head bolts when thee engine was cool again and double checked the spark plug. All are A-O.K
I took the bike out again and did a quick burst of speed with my stock muffle still on the engine. The bike did 35 MPH. I did not hold this speed.I want to break in the engine a little at a time to increase life of the parts.
I was given two 2-stroke muffles and may try to make one of them work on my bike. I would need to make the front pipe connecting to the cylinder block which is going to take some doing.There is a flange plate that pushes the pipe up against the copper doughnut that is recessed in the head rather than bolt up to a flat flange as there is on the CHI-COM bikes.
I'd love to find a used Honda NC50 muffler and do the "guts conversion" but even the used mufflers are expensive.
The tuned pipes you see are nice and the sellers charge a lot for them. Yet you can get a really nice "tuned exhaust" from China for 50 bucks for newer bikes and scooters. Oh well.
LED brake light ( running light on)
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I stripped the engine down and removed the right half. The engine is in better shape than I thought it may be.
Waiting on parts next week.

I got it apart.

There are three kits for sale that will fit my '78 Honda NC50 Express.
 Reviews have steered me away from the Athena kit due to too many listed cracked skirts. I'm not sure if it is poor metal or improper instillation by the owners?
I'm down to the DR kit or the 70cc Polini cast iron kit. Either seem to work well with pros and cons for both.
I plan on smoothing out the ports as needed. I will be using the original head as well.
What have you used and have been happy with or would never use again if it was given to you?
I do not plan to ride the bike for daily transportation,just to retrieve my mail at the bottom of my mountain.
I need a little more get-up-and-go to climb my steep roads.
I've got the Dellorto SHA 15.15 on the engine now and it runs like a champ.
Thanks for your inputs.

Here is an update as of today.
Thinking about what has been said about the muffler being too restrictive got me thinking. If I ran the engine lean,keeping the oil rich enough to prevent seizing,could I clean out the deposits that are in the muffler?
I did just that today. Using the #58 jet and restricting the fuel to the carb I was able to run the engine leaner than normal. I checked the head temperature under full load using a pyrometer and made sure the cylinder never went hotter than 360F,and it didn't.
I drove the bike for several miles and sure enough all sorts of crap blew out all over the rear tire rim. The harder I drove the bike,the better the engine started to run. No more 4-stroking,not even a hint.
I returned home and set the fuel back to normal flow,checked the spark plug and topped off the gas tank and oil.
I drove the bike for about hour at full throttle which was just over 25 mph. The bike pulled as well as the engine could for its size and my weight and my location elevation.
The new carb really makes a difference and If I do the larger bore conversion I'm sure the bike will run well.

I installed the new Dellerto SHA 15.15 today. I have three jets. a 58,61 and the 68 it came with. I made a cork gasket for the carb adapter plate and also used an "O" ring to make sure I have no vacuum leaks at the carb throat.

I test drove the bike and found I have a reoccurring problem.

The engine 4-strokes at high RPM.

The 58 jet allowed me to go 25 mph and then 4-stroked until I went uphill a little. Very slow acceleration.

The 61 jet allowed me to go 20 MPH and then 4-stroked. Pretty good acceleration.

The 68 jet 4-stroked at 18 MPH and stayed that way until I went below 15 MPH for a good distance. Very good acceleration.

I'm told 4-stroking is too much fuel? Or is it too much air? I'm at 7,000 feet.

Could the crank shaft seals be causing this?

The new carb idles very well and adjust as you'd expect it to.

My bike has:

530 original miles on it. It sat for 25 years. Fuel system was flushed out and is like new inside of the tank. New external fuel filter and it flows very well.

Original muffler

Original piston and rings

New spark plug

Timing is a little advanced by using a timing light at the "T"

Oilier is working as it should

Gas is regular and fresh,no alcohol.

Any ideas?

I was looking at this carb Dellorto SHA 15.15 carburetor. It comes with a lever choke or a cable choke.
Have you tried this carb and what choke did you use and why?
Thanks.