You could count the number of teeth on the spur while you don't have much to do on it
You might be able to find out more about all the tuning adjustments at somewhere like
http://www.truggyracers.com/
Sometimes you can get screws out by using a cutting wheel on a dremel and putting a slot for a blade screw driver in the head of the screw. Could be hard if it's a counter sunk screw and you don't want slices in the bottom of your chassis. If not you may need to drill. Do you care if the motor mount get damaged? If you don't drill perfectly straight it will be easy to damage the internal threads in the mount.