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are you pulling the brake? does your brake light come on? the solenoid power comes from the brake, and the ground from the kill switch when turned to off, after it goes through the start switch. do you have a kickstand down? some kill the ignition, some prevent from starting

I just changed out another one, fixed the starting issue, when they go bad, you'll hear a distinctly different sound, as it's not spinning the starter clutch enough to spin out the gear properly, to engage the ring gear teeth. it almost sounds like the engine has no compression when it's failing. the edges of the clutch drive teeth,will rattle against the ring gear. The starter has to spin over the engine really fast, almost like kick starting it, at least on AC cdi ones.

Motley how did you get a long case to fit a short case frame?

That's a myth from the early days of synthetic oil,and no one said go back to conventional oil,you break in an engine on conventional,to allow enough friction for the break in process. Then switch to synthetic and stay with it,

the reason you run synthetic in an air cooled engine, is it can handle much higher heat then standard oils, you only have one break in period , this allows the parts to wear together, such as the rings to the bore, the cam shaft, valves to their seats etc, there is no second break in, once the parts are worn together, they are worn together, even briggs and stratton is going synthetic, you run regular oil for the first 1000 miles and vary your speed, no extended runs etc, the regular oil allows enough friction, for the parts to wear together, you change it at 500 miles, then again at 500 miles, then change to synthetic, the synthetic is so slippery you can't break in parts on it, this is good once the engine is past break in. Also the 139qmb chinese engines, call for 15-40 which is heavy duty diesel oil. the typical powerband you want these engines to run in, is about 8,500 rpm, they have no torque, so you have to keep the rpms up to get it to move properly

it sounds like you are trying to put a long case engine on a short case frame, there are two styles of 139 QMB, the engine is supposed to hit high rpm before the clutch engages, that's perfectly normal, they run around 10,000 rpm normally. the engine will rev up, then the belt will gradually shift from low gear to high gear, and accelerate, but the engines rpms are supposed to stay up, it's a CVT transmission, not a motorcycle with gears. Forget everything mechanical you might know about engines, these are not cars, and they aren't motorcycles, so most of what applies doesn't transfer over. the 5 gram rollers are fine, the normal range on a 139 QMB is around 4.5 to 6 grams depending on rider weight and when you want the variator to move, the weights you have are too heavy and will bog the engine. On acceleration the engine will rev way up, then will stay high while the belt shifts between pulleys to accelerate the scooter then levels off , I don't know whats going on with the jetting but the 90 is dumping even more fuel then the 88 and should make the flame issue worse, I have a 63cc version of the 139QMB,  with a non restricted exhaust, and I can't even run a 90.  As far as your clutch, it's centrifugal it's not supposed to engage until the rpms go up, did you change your contra spring on the clutch? or the clutch engagement springs?   it sounds like it has racing springs installed, is this scooter new? or used? it's possible someone swapped them, if they did, you need stock engagement springs and maybe a blue contra spring