I am faced with the same dilemma. Although my Seagull Zero has a 52 2-stroke installed now, I have a new cowl and would like to install a Saito 82. Even with the 2-stroke, the cowl is too long and the engine is a bit too far forward, but this is not a scale plane and is designed for sport flying.

You might have already seen posts earlier in this thread that someone else installed an OS 70 and had to move the firewall back.