I suggest you do some more assessment of the engine you have before deciding, and before pulling it apart. I would at least run a wet and dry compression test on it. Does it burn oil, or does it leak so much you don't know?

Pulling a head is pretty easy, and after it's off it can be checked for cracks/warpage.

If you get a used engine, you would (should) be putting new oil seals, timing belt, etc. on it anyway. So it's no more work to do that to the one you already have.

If you found a nice low mileage replacement, that would be good, but I bet it's a hard find.

I would not buy and install a used engine if all I needed was a head gasket, valve job, and oil seals on the existing one.

Charley