well max speed is about 20-25mph stock(these are real world figures not these manufacturer ones). you can put a 2 speed and tuned pipe on(30-35mph). as for the engine upgrade, it uses a three screw configuration which screws directly into the crank case. i dont know of any other engine that uses this except maybe a gx15. i tried putting a .21 i had lying around in it but even after heavily modifying the motor mount i couldnt get it to sit in the chassis.

its not a fast car but it handles like a dream. i just use it when i want to play around.