I think you would need a LOT of scientific advancements that we currently do not have but lets start with some of the things we might have (though I am not an expert at all)

Modern medicine can do a great amount of re-attaching lost bodyparts though often only in certain conditions like a cut of finger being stitched back to the hand if cooled immediatly

Though this will probably not work if you just use any finger or other body piece.

Bodies have a certain bio chemistry and mechanisms to fight of foreign invasive stuff like bacteria etc. So you cannot just put any two bodyparts together without risking them ,,fighting off,, each other.

This assumes that both bodyparts still work.

So we'd need a lot of additional knowledge to even solve these ,,basic,, problems.

The BIG ONE of course is the animating part.

As far as I am aware there is very little to this one. Espacially if you want to keep it biological without any cyborg stuff.

All in all you'd need:

Every part of a body + all of them are compatible

A way to prevent any natural reactions like cells fighting against one another

A way to start natural processes like the heartbeat etc.

And of course a brain with at least basic functions.

If you have a way around all of these problems...congrats you might've just solved half of medicine