I had seen a Zuma motor for sale on craigslist and been tempted myself.
Two things about the GY6 will make it very challenging. It is wider than a kit motor. It will be difficult to retain a system for pedaling. The second problem is that the output shaft from that CVT is actually the rear wheel axle. It will be difficult to trim it off.
I think it can be done, but it will require a total chop and weld job in the bike frame.