The best thing to do is to double the firewall.

Make a cardboard pattern (including engine-mount & fuel-line access holes).

Cut out a new firewall of 3/16" or even 1/4" ply & drill the engine mount holes & fuel-line access hole.

Roughen the face of your firewall & epoxy in the new ply on top of the existing firewall.

When cured, cross pin the firewall with #4 screws through the fueslage sides into the edge of of the re-inforced structure.

Fuel proof the new structure with thin CA.

Replace the engine mount & engine.

Check the C-of-G & go flying.