I don't understand why you think the firewall needs moving back. AFAIK, the Saito 120 isn't any bigger (or heavier) than an OS 120 Surpass. The only deviation from the plans/instructions I did on mine was to use two aileron servos instead of one with the torque rods. And my notes on the model don't say anything about balancing issues. I keep rather detailed notes on all my models, so if it had been felony nose heavy, I would have written it down.

Engine, fuselage servos, tank, etc., were all mounted normally. I did replace that egg-shell cowl with a nice glass one, but I undersatand such is included in the kit now.

I have seen one of these fly with an OS 91 Surpass; but "marginal" would be a good word to describe it. I agree with the others, the 120 four-banger is nowhere near being "too much engine".

Steve