139fmb Engine

[BlackPowder]

hello Guys, Im new on this forum so excuse me if the thread aint in the right place.

I have recently acquired a kinroad chopper styled 50cc bike but
here is the catch tho, it came without an engine..
My budget doesn't allow me to blow it all on a nice japanese engine, so one of the chinese clones will have to do.
Been researching for a while now, and it seems china's best answer stands between zongshen vs lifan vs gpx. ( Any brands I'm missing?)

Im looking to start of with a 50cc, then some day later down the line add some performance parts to it ( bbk,cam,crank,carb)
Unless its possible to restrict the top speed of a 110cc or 125cc engine to 25mph? and to fool the cops

So for the 50cc engine Im looking for a solid platform with Loads of aftermarket/tuning parts.

Any help in the right direction will be most appreciated