I recently changed the head on a 125cc quad. When I put the engine back in and tried to start, there was no spark. Far as I can tell all connections are proper, I did notice that now when I turn the key on, the reverse light comes on regardless of what gear it is in. Any advice would be greatly appreciated.

 

Sounds like you have a short somewhere with the reverse light problem. When lights are coming on when they shouldn't be, there may be a short-to-ground somewhere. Check all your wiring for continuity with a meter, it's easy and quick.

It's possible that the same short may not be allowing the key to complete the ignition circuit when you try to start.

Good luck, let us know what you find.