Lungs and hearts are actually now on the list for medtech. The Armory master can sell tools for performing the replacement surgery for 4-5 tokens, and individual synthetic hearts or lungs for one token each. (in addition to limbs, also 1-token each)
(This was where the conversation between me and the AM ended last, it's in the on-ship thread)
That said, there's room for improvement, which was one of the things on my tinker list. I like the idea of a 'combined' functionality. Maybe a synthetic heart that 'breathes' through a port in the chest.
There is also no chance PW will let us haul fully functional robo-bodies in hammer space. It's the kind of thing he's said he's not fine with, and will put limits on it if he sees someone trying it, so we'll have to justify how spare bodies are brought into the field. That said, I am thinking of trying to develop a 'compact' robo-body, which is weaker and easier to damage but lighter, more compact when not in use; easier to carry and store in the field, much like the modifications to the robo-limbs I made which turns them into something roughly the size of a collapsible walking stick, at the cost of them being much weaker when in use. (also listed on the wiki, also under surgery tools, discussion with PW was in the cited IRC log)
After that, I wanted to develop a 'med-bay battlesuit', with the design intention being that the suit (with the aid of a skilled Fleshtech) can perform major surgery including limb and organ replacement and possible revival in the field on its occupant, and carries with it a limited amount of spare parts/bodies for use in the surgery.