Assuming you have a "beam mount" engine? Mount it inverted just like you would a glow engine. Position the mount on the firewall and then measure the distance from the firewall to the cowl opening and add 1/4" (approx.) to allow your prop shaft to exit far enough to clear the front of the cowl. Gas engines don't have the problems inverted glow engines do so, they run just as well inverted as right side up or side mounted. Speaking of side mounting, if I have to cut the cowl for a gas engine, I'd much rather have the hole(s) in the bottom of the cowl than the side or top. The engine doesn't care and it looks better.
RJ