Mine flew good with an OS46AX in the nose. But it could have used a bit more power. It balanced well with that ~18 ounce engine, but it happens that my ARF came with a huge hunk of lead hidden in the fuselage. (Had I known that, I wouldn't have moved the battery into the tail and wound up with a model that was carrying excess weight.)

There was room for the 46, so there would be room for an identically sized OS55AX, which is a moderately more powerful 2cycle. I think I would choose that over the OS61FX unless you really need a 23oz engine up front. Either the 46AX or 55AX would show you appreciably better performance than the TT42 had. They're not just larger displacement but seem to be about the most powerful for their displacement I've seen. I'm doing prop tests with the 55AX right now, playing around with 3blades, and that 55 is swinging some props my 61FX likes.

As for 4strokes..........
The OS FS91Surpass is also a heavyweight at 24oz. Either of the Saitos would be lighter by quite a bit. The 82AAC is 18oz and would give a bunch more power. I've not flown many 4strokes, but have flown that one and it's a HOSS. It'll swing a 14x6 without a problem. I'd go with that if I were doing what you're planning. The 100FA-AAC whould give even more power, but I'd bet it would be wasted. It could swing the same pitch in a prop with an inch more diameter, but is going to weigh 3oz more and cost more.

AS for your tail heavy airplane.........
If there is a lead slug hidden in your Sukhoi, like Phoenix did in a lot of them to make up for lousy wood/design, then punch it out while you're rebuilding. And then move the battery up there. And pick the Surpass if you still need nose weight. And put the lead in front of the firewall instead of on the fuselage floor.

You got 18oz, 21oz, and 24oz engines to pick from, and all of them excellent engines. And plenty of room for them. Also plenty of room in the cowl for ballast if that's needed.