Given the not terribly successful attempts to use fission reactors in aircraft I did wonder about this. Apparently Helion's approach is "mostly aneutronic fusion" - still means that 5% of power output as fast neutrons - which I suspect is probably too much to shield from in an aircraft (mind you I am not a nuclear or aircraft engineer so what do I know!).
Thinking about the fictional Helios in the excellent For All Mankind does make me wonder if it might work in a spacecraft!
But with lots of power output, you can put up heavier craft, even much heavier craft. It's hard to know where all the curves would work out because we have no idea how large, or perhaps rather how small, a mature fusion reactor can be, which means it's more in the realm of science fiction right now than anything we can put real engineering to.
Some interesting actual work on fission-powered planes was done: https://en.wikipedia.org/wiki/Nuclear-powered_aircraft but as the article says the niche they would theoretically occupy ended up going away, especially since a crashing fusion-powered plane is a pretty significant inconvenience but a crashing fission-powered plane is a catastrophe. We're not used to thinking of air flight as something with huge amounts of power available to it for very long periods of time, so our intuitions don't match what is possible. Still, I would think these would be very expensive, if for no other reason than that there is no reason to build them unless you meant to make them very, very large. It's the sort of power plant that could make something like the helicarriers in the Avengers movies possible (albeit I suspect with rather more surface dedicated to control and pushing through air) and it would probably have to be a plane, not a helicopter-based system), but even if we had the blueprints in hand we'd be looking at investments sized more like Navy ships than airplanes.
