Re: 50cc GY6 on a Bicycle??
The biggest hurdle to using the common scooter mill is the chain drive since most are setup for a scooter drive wheel. The key is to find one of the versions used on the 4 wheelers since they already HAVE a chain drive conversion box.
Using this version, all you have to deal with is the extra length!
And BTW, the HUGE number of available upgrades for these motors can make them quite powerful and rawkus!
If more is better, then too much is just enough.
If you can't afford it, build it yourself.