Actually you did pin the former in. Pinning the firewall is basically the same thing. You could use toothpicks, 1/8th inch dowel rod (my favorite) or wood screws.

Basically, if you use the dowel rod you drill 1/8th inch holes on all sides of the firewall, I'm talking about the piece of plywood that you actually mount the engine to, make the holes about 1/8th to 1/4 inch deep. Now put some 15 minute or longer epoxy in the holes and on the dowel rod and stuff the rod into the holes and wipe away the excess glue. Cut the rod flush and there ya go. On the Cap I did 3 each side. I base it on the engine size so most 50cc engines get 3 per side, 100cc engines get 4 per side and 150cc+ engines get 5 per side.

If you use toothpicks then the hole should be the same size as the toothpick with the pointy end cut off or if using small wood screws then no longer than one size smaller than the screw itself. Use epoxy to help hold in the screws as well and certainly use epoxy with the toothpicks.

I wish I had some pics for you, it would make it easier