Kev
hello, and welcome to the forum. The rail bike is super cool for sure. Would be cool cruising along the tracks.
Have a couple of bmp kits. Very happy with them. Amazing low prices.
You will have proplems with a weed wacker motor. Most of them have a 54mm clutch and different bolt pattern for mounting the motor to the mount. they are short on power imho.
Am fairly sure all motors that have a 76mm clutch will fit the bmp kit. The throttle and kill switch price with kit is a good deal. The motors are often hard to stop with out a kill switch.
There are some post or/and threads with nice build`s (friction drive) using wacker motors. Not sure what problems when trying to fit bmp drive shaft to wacker clutch drum.
I have all 49cc motors on my mab`s.
Good luck, have fun!
Ron .