ORIGINAL: DarZeelon
Jerome,
This model is designed to accept a full-size .60 engine.
This goes both for size and for balance (C/G matters). The Evo .60NX is a .40 size-frame engine and also weighs as much as a .40 engine, and so is the OS.55AX.
So, both of your choices here are 'wrong', as far as the designer/manufacturer of this model is concerned.
You bring up a good point, DarZeelon. Although I'd say "both of your choices here are 'undesireable' " instead of 'wrong'. Iwill keep your point in mind for sure.