Re:
Certainly no problem actually replacing headtube. Cut out and fit a new one like I did with the seat tube.
Problems arrive with the real need to post-weld heat treat and the consequent risk of over-treating the existing frame welds. Otherwise, might need to anneal the whole thing post-weld and start again.
The jury seems to be our on whether it's necessary for 7005 repairs, but it is pretty much a requirement for anything 6061.
All the best,