[What's_nitro?]

1. Would a Baja pinion gear be the right size? i can't find the I.D. of it anywhere

I don't know. :)

2. Would i be able to do a direct drive from the pinion to a spur and out to the tires, without a clutch?

A single reduction could work. You need a clutch...otherwise the wheels spin as soon as you start the engine, and would probably cause it to stall a lot.

3. Would cutting the engine off work as a decent brake? i'm not looking for much and i wont to keep it lightweight

Yes, if you only want to stop once.

4. What kind of tires/driveshaft could i use?

I'm picturing a giant pan car in my head... Any wheels/tires could be used depending on which hub adapters you put on the driveshaft.

5. What kind of speed would that give me and would it have enough torque?

With a single reduction you could go 20-50 mph... Acceleration will probably be slow since it's not a "performance" engine but it'll work.

6. Would i be able to use a steering servo that doesn't cost much?

How much do the Baja servos cost? You'll probably need something just as rugged.

7. Could i even build it only using a drill press, grinder, and my friends band saw that can cut metal?

Probably. What material(s) did you have in mind?