Make a six sided box with epoxy glued angle stock gussets along all the joined edges. Make it as deep as you need it to mount your engine. Epoxy and bolt that box to your original firewall and then bolt your engine to the front of the new box. Cut access and/or lightening holes in the sides of the box as needed. I would use 3/8 inch thick aircraft ply for the box, NOT light ply. Good luck.

Regards,

Richard