You can just get a replacement motor and use your current motor to learn about these motors. boygofast on eBay and many others sell just the motor usually for less than a hundred and I have found that the aftermarket parts for these motors are of a better quality than stock.
The clutches on these things can be real tricky to set up and can cause the problems your having. The clutch release needs like a preload that's difficult to set and under the cover there's a "star" nut is a cause for problems, too.
There are good tutorials here, learn and enjoy the hobby.