Oh...!

If you've got the depth to do it, make a 2 stage motor mount. Then you can move the motor around on the second stage all you want until it's where you want. Mark the mount holes and drill.

On your Mossy, you could make the first stage tall enough so the motor sits on the second stage (no stand-offs). Temporarily make it tall enough so the prop shaft extends out the cowl. Drill a big enough hole on the intermediate mount so you can slide the the motor around.

Then, draw a circle the size of your cowl opening on a piece of light cardboard (cereal box works well). Drill the center the size of you prop shaft. ( You could possibly use your intended prop spinner base as well.) Put it on the motor, and use it to center the motor in the opening. Once positioned, remove cardboard, mark and drill mount holes.

If the first attempt is off, no sweat, chuck the intermediate mount, and try again. That way the firewall won't end up like swiss cheese.