Simply use some layers of plywood or hard plastic sheet across the firewall for the engin mount to satisfy yourself.

As explained by others above, that is a bad choice. The Kadet needs down-thrust.

Nay, not so. The angle of attack (AOA) is the free air flow measured against the wing's chord line, not just the prop blast. Notice that a normal flat-bottom wing usually has a rounded leading edge. The center of that LE then sets the chordline which already sets some incidence angle reference the bottom of the wing itself. Given the stab is zero relative the wing bottom, simply by the LE radius, there is some incidence angle set-up.

With a straight engine thrust line, you will have to trim the elevator to maintain straight flight at any gven airspeed. Pull off power, and the machine will dive. Add power and the machine will climb as it seeks to maintain the airspeed that you trimmed for.

Induce downthrust and now when you trim for level flight all is well-- and when you pull the power back, the trim offsets the thrust that was pulling the nose down, thus the machine will tend to maintain the flight line established prior to reducing power. By the same token adding power will create more nose-down against the previously set trim, thus reducing the tendency to climb.

Any flat bottom high-wing will do this. Down-thrust is the answer. The original SIG Kadet was the main trainer of the '70s. SIG did not show downthrust. We had to install it for the newbies. Now that SIG is run by real modelers, I think they understand the basics.

I suggest you go with the aerodynamics rather than the cosmetics. A pilot only needs to know the lift formula that HE can control, which is reduced to Lift = AOA x 1/2 V squared. (Lift = coefficient of lift [AOA] times 1/2 of the air mass density, times velocity- squared, times the wing area.)

Use the downthrust.