Some older notes here, since I actually did the swap a few weeks ago but wanted to share:
I bought a broken Mylodica from Reverb, in retrospect wasn’t the best deal but I think it worked out. When it came it had two issues relative to new: the E4 reed was broken off (and everything fairly rusty in general), and the wood around the bottom two screws had stripped out.
The bottom panel was an easy fix, I used epoxy with wood flour since I had it handy but I imagine bits of toothpick and woodglue would have served just as well. One the filler was starting to set in the holes, I pre-drilled just to be sure the hardwood didn’t split, and it’s on quite solidly now.
As for the reed block, I can confirm that the Angel-branded “Melody Horn AM-37K3” melodica I found used on eBay did fit my broken Mylodica perfectly. (Likewise, the old innards went back into the green plastic Angel body just perfectly, including the spit valve. Gifted those leftovers to a young niece who hopefully will enjoy the remaining 36 keys 🙂
Unfortunately the Melodyhorns don’t seem available new anymore, though I found a few potential candidates like the Youngchang YM-NK37 (Korean-made as the Angel was, but different spit valve arrangement) and some unbranded China-manufactured 37 key instruments that looked similar. Hopefully I never need one.
It now works well and sounds good overall, though I’ve been using a new Suzuki as my “daily driver” since that one has a spit valve. At some point with the Mylodica, I’d like to add some rubber feet to the bottom for two handed playing, and perhaps work out some sort of spit valve lever — frankly I’ve yet to get much spit out of the bottom of any melodica anyway so that’s not going to be high priority.