Changing the valve seat is not an easy job. I've seen plenty of engines with new valve seats, supposed to be needed because of use of unleaded fuel, release the newly fitted seats into the combustion chamber.
The installation requires real machining skills, Nitrogen freeze, negative tolerances... The kind of skills you could acquire when it was common to change the valve seats on engines, that is to say the kind of skills that you can today almost only find on retired mechanics.
Considering the price of a used head, and the chances of fail, I'd consider buying another head.
Maybe you could find shops specialised in overhauling K heads at Mini specialists, who love to adapt these heads on Mini engines.