Re: 50cc GY6 on a Bicycle??
You haven't given much info on how the bike is configured. Generally there is a jackshaft required to transfer the drive to the derailleur, and the ratios are very important as L2 has indicated.
I am assuming you are running the 4stroke engine. Depending on which kit you bought you may have the one with the 3:1 pocket bike trans. This kit has too little primary reduction, it should have a 5:1 reduction to make it work well. I have one of these. When I bought it I assumed a 5:1 box. They don't tell you its a 3:1. Maybe this is part of the problem?
I have a ton of hands on with a 150cc Gy6 in a Honda ruckus clone scooter. I extensively modded the engine and variator. It will easily show the north side of 70mph.
They are wonderful powerplants. However for modding to a bike it will require some pretty good fabbing as its an engine/trans unit that is also the rear suspension. I suppose the rear triangle could be removed from the bike and the Gy added to the frame some how. But running a scooter sized wheel would be required as the final ratio would be way out of wack for a bike wheel. Not trying yo discourage you if you have the skills go for it. Just pointing out a few facts.