Yes the .90 would be way over powered. I've done 4 of these and they had from a .46 2-C to a twin .90 4-C. The most popular engine is a .70 4-C and even that is over kill especially if he wants scale flight. You will see 2 that I did on my web site, one the WW-II had a Magnum .45 PRO which was a good engine for it and the yellow one had a Saito .90 twin in it. The twin flew like a rocket!!!!