the c20-2050kv motor I am using has the fiberglass motor mount on it. The firewall has pre-drilled (slotted, rather) holes in it. The holes to fasten the motor to the firewall are outboard of the slots, should I drill into the fiberglass motor mount of the firewall? The slots are about 1/3 of the way into the holes on the fiberglass mount. I would think that drilling new holes in the fiberglass would be the easier route as opposed to trying to extend the slots on the firewall. The fuselage is already built so would drill the firewall by hand.

I'd plug the holes in the firewall and drill new, for the motor mount.