Simple items

I think one of the best solutions would be to make screenshots of the rendered weapons, and placing those on a sprite.
The "damn small xonotic" version might already has those sprites, and only the textures needed to be replaced with higher quality ones.
Though like I said, simply replacing only those models is not enough, as it permanently replaces models (not configurable), and also, it's altering of core materials, hence considered cheating.