1. It shouldn't be. If the new motor, pinion, and spur gears were installed correctly.
2. Are you running it in a mode where you can go from forward directly to reverse? Then, maybe. But it should be set for forward/brake/reverse, where you initially get brake on the first pump of reverse before it actually goes into reverse.
3. Yes. You need to know what the gear pitch of the spur gear is, and match the pinion gear to it. Is it 48 pitch, 32 pitch, Mod1? What? It's crucial to have these two items matched.
4. First, I'd try going to Hobby King's website. They seem to have all the documentation, and even discussion forums. Or you could phone them. Otherwise, there are several RC race tracks in So Cal. Orange County RC comes to mind. Look up hobby shops in L.A.
Sounds like you know the paper method for setting gear mesh, so you're doing something right. Did you remember to put loktite on the motor mount screws? That's important too.