I have a modular weapon and tool mod that uses item modifiers to swap out parts like heads, blades, grips, etc. I found that I have to add a box collider to the base item around where the blade will be for hits to register, and make sure that the item modifiers do not have colliders. Ymmv, but I have tested this extensively.
Just picture an axe handle with no head but a box collider around the end. The various axeheads are cosmetic and add dmg in the xml.