I have searched the forums a lot. I just bought the Flying Horse 4-stroke 5G motor from spooky tooth. I am looking for a bike under $150.
My condolences.
That engine has a 5/8" long shaft, you have to mount the clutch itself AND the clutch bell itself on the same shaft, with the bell spinning on a crappy brass (oil-lite) bushing that even with frequent oiling wear out.
I build with HS 142-1G taped short shaft engines with clutch attached and 4G transfer case with the clutch bell attached to it.
The bell is on it's own dual bearing shaft.
The best kits are here
http://www.bicycle-engines.com/4-Stroke-Engine-Kits/
$150 for a bike?
You aren't going to find a good bike for motorizing for that unless it's used.
The Macargi Pantera 7S though tight, is a good bike for ~$240 with dual V-brakes and 7-speed.
The back sprocket mounts to the 7-speed hub just about perfect, and you can set you pedal gears for the right gear to make pedaling easy as you go.
I am into building shifting bikes, which means the engine can use the bikes gears.
Gears change everything, and no left side chain.
A good bike for this is a Fito Modena 7-speed.
3" wide balloon tires and dual disc brakes.
The bike is $350 delivered and pretty awesome, just not quite as awesome as changing out it's frame for a gas tank frame with alot more room.
Gears allows that little ~50cc engine to get to 20mph in a couple seconds and top out ~40mph with the braking power to stop you just as quick without ever having to pedal (though you can).
Anyway, hope that helps you look at your options despite having a low end failure prone kit to start with.