Looks like you have about 32 teeth on the axle sprocket, that's really small. Thinking that original motor would have been one with built-in gear reduction, or some other thing I don't have any experience with. GY6 is a common Chinee engine, has built-in tranny, don't know much more about it. If going straightforward engine style like a Predator 212cc, you'll need to get a larger axle sprocket, like a 60-tooth, to go along with the 30 series CVT (belt-driven Constant Variable Transmission, aka: TC, or Torque Converter) you'll need to put on the engine.
Forget the original setup of engine, it's been changed up, doesn't matter. You can put an engine on any way you want to, (drill some holes/cut out some adjustment slots), bolt it down, change the axle sprocket, break a piece of chain to desired length, hit the gas.