Honda 400 engine advice please (Honda Falcon)

[Sheonagh]

Currently on a couple of Honda Falcon NX400's (Think NX650 format with XR400 engine, made in Brazil) doing a long Tour (Central America, Far east and keep heading west...) and doing a little routine and other maintenance in Los Angeles.... My Questions
Who in that area might have a lot of experience with XR400 engines...?
When I took the spark plug out of one it was very stiff and brought some alloy with it, the new one has gone in and tightened down but I wonder if I should pull the head off and helicoil the plug thread?

• What would cause a plug to seize / damage threads. Possible overheating?
• should I leave well alone or recut the thread/ helicoil? There is like 11 turns of the thread and perhaps/ the first 4 may be a little dodgy now, is it possible it might spit its plug out if I leave it any experience? Is There enough metal to put in an insert?
• removing rocker cover... it is likely that I overtightened the small bolts last time I had it apart. If I encounter damaged threads is there enough metal to helicoil these..... any experience
• How long do these engines last ( big ends etc) they have been regularly serviced. Fed oil, had timing chains replaced. Are not obviously using oil between services..... have now done 56000km (35000 miles) any experience?

Any advice would be very gratefully received.....
Sheonagh (and Pat) aka 'Dusty old bags'
PS anyone got a spare crate near here ?