What if you made an aluminum backing plate large enough to cover the engine mounting holes and use PEM nuts in it. You could set the PEM nuts using a vice and then slide the backing plate in place. Of course, that's accuming you have access to the back of the plywood engine mount. You can find info on PEM nuts at [link=http://www.pemnet.com/fastening_products/prod_desc.php3?cat=Fasteners+for+Sheetmetal&df=nex t&sp=16]http://www.pemnet.com/fastening_products/prod_desc.php3?cat=Fasteners+for+Sheetmetal&df=nex t&sp=16[/link]

What you would want is on page 2. They are type CL. It might also work to use PEM STUDS (on page 1) with elastic stop nuts (available at ACE Hardware).

Good luck with your efforts.