Jason,
There is no problem in threading your nylon mount. Use 6-32 or 8-32 machine screws (cap screws for allen wrench), whichever will fit through your engine mounting holes. Stop short of threading the holes all the way through, leaving the last little bit of the hole unthreaded. This acts as a lock to keep the screws from backing out. I use lock washers in addition to this just as insurance. Don't use lock-tite on plastic, it attacks some plastics. I usually use a sharp scribe to mark the location of the holes, but Great Planes makes a pilot drill gadget that works well on the nylon mounts, too.

Pilgrim