Hiya.
Yeah, the .46 will be well underpowered, - I know, as that's what size I first tried mine on before removing it and getting a .91 FS, which in my opinion is just about right.
One or two others on here have got .90 two strokes in theirs and are happy with them.
A 1.20 is a bit big, but you might get away with it, as long as you use careful throttle management to keep the airspeed down, as its big control surfaces are prone to flutter at speed.
Weight wise, you should get away with 650 grammes with your 1.20, as my .91 FS weighs 742 Grammes, but I do need a bit of lead at the tail end to balance it !
If it was me, I'd probably go for it and try the 1.20 rather than spending more on another engine if you don't need to.
(which is why I tried my .46 to start off with, which turned out to be poor)
All it'll cost you is a bit of time and an engine mount to find out ?