the epoxy i mentioned, jb weld, is a cold weld epoxy. it creates a very strong metal on metal bond. proper application should render it about 95%+ invisible. would only need applied to the flats between the pieces. there is enough of a hook on the mekugi ana to hold the blade in the tsuka without the rear piece. thats why i am thinking it would be the best way to go. the corrosion levels on the blade and nakago had me thinking even later than that. maybe the 30s or 40s. its mostly surface corrosion with mild pitting.
*edit - now that i think about it, i was judging the rust levels on what occurs in my area. i have know idea where this has resided before california. its hot, humid, coastal air here. makes everything corrode quickly.