When changing to a larger engine, you need to consider more than just engine weight. A Saito .91 weighs only 20.5 ounces with muffler on my digital scales compared to 16.5-17.5 ounces for most .46 engines (not the Rossi, which is heavier). But, you need to factor in a heavier motor mount and a heavier prop. All that being said, I have a Goldberg Extreme 330 that was going to be tail heavy by a few ounces and the big Saito balances it perfectly.

Most modern .61s and the bored out .75s and .91s in the same case weigh in at 25 plus ounces so a Saito .91 or even a Magnum, 22.5 ounces for mine with muffler, are a better choice. You will need a longer gear. You might need to mont both the elevator and rudder servos in the tail. I am using my Big Stick 40 for an engine break-in plane for .46 size engines or I would have done this mod myself. Mine has had an OS AX, a GMS and a couple of Mag .52s on it already.

I would fiberglass the firewall. The 4-stroke should use less fuel than a 2-stroke so you might want to use the same tank. I also would add 2-56 bracing wires on the tail. These will strengthen the tail and add a little weight in the rear.