I used to motocross and now have 2 sons starting to ride. It's been 30+ years so bikes have changed a lot. I just bought a used 2004 KX65 and it starting frying before he made one lap on a little track. He over-reved and it locked up just like he was standing on the break. It started back up with a mild vibration when idling down. I let him ride again and it did the same thing. This time with a heavy vibration when idling down. We loaded up and went home..... I'm I looking at top end rebuild or crank. Any idea's and any pointers on how to do as cheaply as possible. I've got 2 of these things to keep up. Thanks for any input.