I was able to put in a normal Hitec 525 MG servo. Don't like those tiny little servos. I've seen too many of them burn in the middle of a race.

I just filed the posts enough to put it in. You can also enlarge the holes on the chassis a little bit so that the posts can accomodate a bigger servo



Oh yeah, it's difficult to fit the motor. If you put a schottky diode in some motors, the best way to change them is to remove the top plate.
It's much easier.