first of all, i would like to express my opinion on those bikes.
they are not impressive builds at all.
the engines look like they are the same ones you can get out of a 49 c.c. mini chopper.
look up roketa and you will find this style of engine, which is based on an old honda design. (horizontal cylinder.) it appears that he used a roketa (or honda) horizontal 50 c.c. 2 stroke, and then mixed in some h.t. kit parts (the gas tank, the throttle and maybe the rear sprocket and chain.)
the gas tanks on these bikes do not look good on that frame and i just do not like the way these bikes are set up...they don't have peddles, so they are no longer bicycles, but the drive chain is on the right side, and he is only using front brakes.
if the guy was good, he would have made the original jesse james gas tanks functional, which would look way cooler.
I have a j.j. chopper but it is not motorized. there really is not a "right" way to put a 2 stroke h.t. engine on one of these bikes because of the way the frame is, without doing major surgery to it. (the frame).
i suppose a 4 stroke could be mounted with a flat plate of some sort, but to be honest, it would probably look like s**t!
i do not see a way to mount an engine to this frame to make it look good. (like a "real"motorcycle or mini chopper. i mean one could be butcherd up and welded back together i guess.
again, just my opinion