Honestly, you could probibly mount a .90 two-stroke with out too much trouble. You might be able to get a 1.20 in there with a little work.

Of course, you wouldn't want to do this for training, and you'd be putting serious strains on the structure. It would be really easy to rip the stab off the plane or fold the wings.

There are a couple of guys in my club with .90's mounted in .40 size mustang ARFs. They are FAST, but these are really good pilots and you don't see them doing much more than high speed passes, some rolls, and some very very big, open loops. You could do the same with an LT-40, I suppose.

I flew a new LT-40 this weekend for a new student. He had an OS .52 4-stroke in there, and I think it was just about perfect for that airframe's intended uses.

If you're just learning to fly, go with a good .46, or if you must have a 4-stroke, a .50 class will work very well.