I forgot a small item that may help out a little (just a little. You can add a dowel to the plywood plate that you made to match the engine backplate. Make SURE that the dowel is located exactly at the engines prop shaft center line and is dead straight with the plywood mock up.

If you have also drilled holes in the mock up back plate that correspond with the mounting holes of the engine back plate, you will have a complete package for your engine installation.

Center the dowel in the cowl, mark you mounting hole locations, and voila! Your done.