I have this exact problem on a 2005 TXTpro 250 that I got cheap off my brother-in-law. I have a friend with a machine shop so was going to take the head down to him to sort it as I'd read that helicoils (my original plan A) aren't the best job for this sort of repair.
Or would I be better to just take the whole engine out to reduce the number of gaskets etc required.
...and while I'm hijacking the thread, Is there a go-to UK or Ireland distributor for gaskets and the like?
Thanks in advance - I've only just found this forum and have learnt a great deal already!