Re: 50cc GY6 on a Bicycle??
It's all doable if you can weld and either make a custom frame to fit the engine/trans or cut out the lower frame and make up your own mounting setup. The GY6 engine would make for an interesting build tho since there's a lot of upgrades for that engine and CVT.
If you plan on using the standard 26" wheels you may need to make up some type of jackshaft since the rear of the CVT is setup to connect straight to the rear wheel and I don't know how much clearance you will have, but if you got welding and fabrication skills you should be able to get past that hurdle too.