HI
GO WITH AN OS 55AX- any plane you could put a 46AX in will take a 55AX so why not go for the added performance you will gain remember -performance is always a power to weight ratio i do think a 55 is a board out 46 so the engine weights are about the same and the power of the 55 is greater- in fact the power of all the AX series of engines is awesome- my 75 AX is turning a 14/6 apc @ 11,300 RPMS my 91 FX turns a 13/6 apc - a full one inch bigger prop- all AX series engines are putting out more power for there size than any other series of OS engines in the past and that is a real good thing
A 55 SIZE engine is as big as i would go in a 40 size airframe-or the wing loading starts to be an issue especially on landingwere you need to look you best always