Yes, you can swap in a gy6 50cc. No you can't swap in a 125cc because the motor mounts are in the wrong place. But the 50cc swap is a no-brainer. Everything fits except the inner rear fender liner, and that takes just a simple home-made bracket to make it work. Another commenter says you have to swap over your old Kymco ignition assembly (behind the flywheel) to the new engine. This is correct, but it's not difficult, maybe takes about 1/2 hour.