4s Rustler, which motor should I get?

I think the best pinion solution would be to order the right pinions from Mike. If that isn't possible, for what ever reason, you can drill out the holes. You just have to make sure that the drill bit goes in straight and centered. I did a couple using my lathe and a collet. Of course, it also depends on whether there is still going to be enough meat for the threads of the set screw.