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I can`t help unfortunately but I`d be very interested if someone else has encountered this and found a solution.
I've had the same problem with a couple of 125`s on the fleet, both had been run very low on oil to the point that only a couple of drops had come out when the drain plug was removed. I tried adjusting the valves, checking compression, adjusting the carb, even replacing the carb with a brand new one. Eventually I replaced the engine on one and put the old engine aside for parts. The other scoot ran similar but was useable so I gave up and put it out for hire, so far no problems have been reported.
The only conclusion was that maybe either the cam chain had stretched or the crank had bent slightly or become unbalanced. I must admit that the bloody things almost had me tearing my hair out at times.

Out of interest Alley, do these kits need the crankcase relieving in the same way the 170 kits do when fitted to a 125/150 engine ?

Nice tutorial. I used to work in an Aprilia dealership and we regularly got scoots coming in with crank bearings blown because the tiny belt had snapped. The owners had been putting 2t oil in as they were supposed to but had never had a service done.
Whilst a good idea in theory, I can`t think of a worse idea than to have a rubber belt running such a vital part such as an oil pump, especially on scoots ridden primarily by teenagers.

Magnetos on mopeds are old school now and generally have contact breaker points which needed adjusting to the correct gap, and a condenser which stores energy to give a spark through the ignition coil.
The modern cdi system uses the pass over pick up on the flywheel which sends a signal to the cdi box which then sends a signal to the coil.
The essential difference is the way the spark is generated ie: contact points/condenser versus flywheel pick up/cdi.
The contact points were a pain as they needed regular adjustment, and as they were constantly moving there was a good chance of heat/wear build up, sometimes leading them to welding themselves together.
Modern twist and go 2 strokes are exactly the same set up as modern 4 strokes in that they have the flywheel pick up, cdi, and coil.
As far as ignition coils go, although they may look different they generally serve the same purpose, have two connections and a high tension lead.

Very fashionable mopeds in certain circles. Here`s a few links....
www.mopedarmy.com/wiki/Category:Motobecane
www.mopedarmy.com/photos/brand/1/
www.mopedarmy.com/wiki/Motobecane

The only way of telling is to take measurements unfortunately. Most of the Ruckus on the forums have a GY6 150 engine as a base. Yours being a 50 has different mounts to the 150. Have a search on there for 139qmb, it should throw up a few 50cc bikes