Re: 50cc GY6 on a Bicycle??
To me, the variator drive is worth a lot of hassle to get into a bike! Even the 50cc is easily good for 50mph with the variator using engine power to best advantage.
The chain drive ends up with close to normal gearing on the four wheelers, so it's not too far off what you need. Some careful gearing research should make this a very user friendly engine choice!
And like I said, with the aftermarket hop ups and big bore kits available for these engines, you can easily get a seriously fast powerplant!
If more is better, then too much is just enough.
If you can't afford it, build it yourself.