Can I take one of the vex motors and make vent holes in the side?
If you were not using these motors for competitions, it would be okay for you to do that at your own risk.
I don’t think that you would be able to use these in competitions however, as per stated below.
<R13> Parts may NOT be modified. a. Examples of modifications include, but are not limited to, bending, cutting, sanding, or melting.