If the spark plug has been x-threaded, there's no alternative to a helicoil I'm afraid. The internal pressure will blow out any bodged fitting (and you may be correct about swarf in the cylinder).
So, head off I'm afraid.
You can try a used one but how can you be sure that what you're getting is going to be any good?
Best bet is a local breaker or someone on here who has a spare head.
GeoffT in the Orkneys was looking for a 550 head recently. I have a complete 550 engine but shipping it up there would be prohibitive.
seen one on ebay item no 110231895640
says it needs work but will cost 20 inc shiping
dont realy know about this sort of thing ..
dont know what a helicoil is . im guesing that i will need to take it in some were ..
as for your engine . i live in cardiff at the moment . but do trips back home to hull and manchester a lot . the bike is never going to be a show winner and has been altered in a lot of ways so i am looking at the cheapest option ..