Depends on what kind of "fix." The engine was a BMW and BMW would look at 'em, I guess. The McLaren F1 was never officially imported into the US, however, so McLaren had no legal obligation to have anything to do with them.
My motorcycle is one of twelve imported into the US. The dealer network they had was primarily Harley shops; there's nobody there who can work on them. I know of a shop in the UK that will work on them, and I deal with a shop out of Nuremberg. If I want a mechanic's opinion, Skype is involved.
Granted, it cost hella less than a McLaren F1.