I can't figure out how any of the engines that will fit this plane can "loosen the firewall to the motor box". The motor box structure extends all the way back to the wing saddle, and is cross supported on top of that. It's built reasonably light, but definitely strong enough to hang a 4 cubic inch or larger engine on it if you could get it to fit. And balance, which it would not.

I'm extremely familiar with the construction methods used on this plane. Since I had the time, and that I had some ideas I wanted to look into I removed a very large amount of the covering on the plane. The full bottom covering, one side from the nose to the wing saddle, and half the front side opposite the one that was removed to the saddle. The Ultimate is built very well!

Pat