First of all, I would be madder than a hornet, myself - especially if the shop was an authorized Yamaha dealership. If it was, I would get in touch with Yamaha factory or reginal sales mgr - they should help you on this.
I would inspect the threads on the plug to see if they are stripped, or if the housing threads or stripped - if you are lucky, it will be the plug threads.
Don't know if an oversized plug is available or not, but I think a helicoil would work - try to find a marine mechanic who has done this before.
Good luck [img]smile.gif[/img] ,