[fly_away]

I wouldn't expect putting screws into foam to do any good at all.
You'd be better off drilling some holes (say 8mm or more in diameter) back into the fuselage about 30mm or so and gluing some wooden dowls or pegs down into the holes using plenty of adhesive. Then put screws through your ply mounting plate and into the ends of the dowls. It doesn't look like a very substantial plane, so perhaps two dowls would do it. Plenty of glue on the back of the ply plate would obviously a good idea too of course.

Actually behind the foam is the solid fuselage. Machine bolts with nuts would be ideal but I dont think I can access the back of the firewall. I am thinking of stripping the foam from the firewall and glueing a block of wood. I can then screw the mounting bracket to this block. Yes it is very cheaply built.

Interestingly I heard from someone that engines in commercial jets are simply stuck to the wings by araldite and meant to break off!