The hole under the fuselage i shall be covering with white covering just so no oil enters the plane from underneath.

The pictures below are of the firewall, it was hard to get good pictures due to engine already mounted but i will try and explain how i set it up.

In the first picture the original firewall is still there i only cut the electric motor box off flush, the engine mount beams are sitting on a new firewall i added.
Once you cut off electric motor box, you can hold a square bit of ply up to the original firewall and trace around the fuselage to get the same shape. before i glued the firewall to the other original firewall i first created a doubler which you will see in the second picture. there is a slight hole in the original firewall which i filled with a cutout bit of ply, the one with the barcode still on it. but even with that sitting flush inside there is still the gap from where the electric motor mount was as it is hollow. i shall try and draw a picture in MS paint, obviously it will not be all that shabby lol. If you dont fill this gap there will be a square the size of the electric mount hollow between the two firewalls and doubler, creating a firewall that may warp.

HOPE THESE PICS HELP :P