Welcome to MBc, Caveman. Now that you have introduced yourself, be sure to read all you can, including the 'Stickies'. They contain a lot of information. For example, Sticky #1 under ‘Introduce Yourself’ has 20 or so links to “How To…” threads, and important pages for you to follow. Sticky # 3 has info on forums, how you are expected to conduct yourself, and where you might find and use various features. But, read all of them, and be sure to use the search feature. If you are still stumped, post a question in the proper Forum. Good Luck.
I wish I could help you, but I don't know a lot about mountain bikes. My favorite motor, however is the Robin-Subaru EH035 4-stroke. For its size, it is a potent little powerhouse...but it is not cheap, I'd start my research. Remember...if you start with a cheap bike and cheap motor, you will end up with something un-reliable, UN-safe, and costing you a lot more than if you would have started with a quality bike and quality motor. Again, Good Luck. (p.s. Keep the 'Barney Rubble' brand products, Caveman.<g>)