To answer your question, going from the .46 to the .55 is a torque upgrade. You'll see pattern pilots with .40 size planes or very light .60 size planes use it. You can turn a bigger or higher pitched prop with a .55 instead of a .46 at roughly the same RPM. The .55 was designed for pilots flying .40 size planes who wanted more pep without having to go to a tuned pipe exhaust.

Since everyone else is offering advice, I will too. Don't move up to the .55 unless you simply want to let them run as much speed as possible. Pick a powerplant that is plentiful and inexpensive and stick with it. Both of these engines qualify there, but the .46 will give them a lot to work with. Don't limit the RPM either. As stated above, the porting of the engines won't allow them to turn fast enough in the air to damage themselves anyway, at least not with a prop that is actually usable on a real airplane. If you're really that worried about it, go ahead and spec a realistic prop for them to use.

All that said, I'd reconsider abandoning electric power. The big issue you're going to have with glow is having the teams learn how to tune glow engines. It's not really that hard, but just 2-3 clicks in one direction or another will take a plane from a top contender to an also ran. You've been around pylon racers so you probably know how much effort they put into engine tuning. You'll be adding yet another set of skills and variables to an already complicated task by going the wet power route.

If the issue is broken prop shafts, buy motors with replaceable shafts and keep an inventory of them in stock. It is so much easier to level the playing field with electric, because you can simply set a watt limit on the ground and be done with it. Teams can play around with their prop selection and get the one that works best with their airframes without going over the limit. It takes all of a minute to put a watt meter on the plane and measure the current, so there will be no question on race day if the plane is legal.