If you have any old brushed motors around, the screws that hold the brush tag to the brush hood will do the job, if it is long enough. I think those screws are M2.5, but I've used them successfully in the end of servos for years. Just don't overtighten them and it'll work fine.
If you've no brushed motors left, or friends with them, then search on line as they are not difficult to find. HTH
