For a conversion the firewall is the heart of the project.
You have already been given good advice about how to figure out where you need to put the firewall.
i'll just add that for a small engine like the 20FP you only need 1/8" or 3/16" thick aircraft grade plywood for a firewall. Use #4 sheet metal screws to hold the plastic engine mount to the firewall.
The engine should be mounted to the plastic mount with number 4 [#4] sheetmetal screws as well.
Choose a drill size to make snug fitting pilot holes for all of these attachment points.
I recommend buying a Harbor Freight "Numbered Drill Index" so that you will have a great selection of drill sizes for this level of model work and also a cheap Harbor Freight dial caliper to help you "size up" good matching pilot drills for your screws.
6 in. Digital Caliper (harborfreight.com)
Titanium High Speed Steel Numbered Drill Bit Set, 60 Piece (harborfreight.com)
To cut 3/16" thick plywood you can use a hack saw blade if you have nothing else..and course sandpaper to do any shaping you can't do with the hacksaw blade The plywood will need to be backed with firm balsa wood.and even balsa saw dust mixed with epoxy..where it meets the fuselage.
If you could take the time to study some published .15 to ..25 sized model plans to see what the typical design details are then this will be more help than what I can offer.