Had this happen when I had the stock 40 horse, before she cacked. I dropped the motor and was gonna pop the head but then talked to Peter Roberts (in Vic) and he had a nice little kit that is apparently way better than a helicoil. Just greased up the tap so the filings wouldn't fall into the cyl, chased it, threaded an insert into the head, with a little locktight and then had a tool that cut a new face into the aluminum for the spark plug gasket to seal on. I guess you could do it with the motor still in the vehicle. Someone in Van has to have this same set up.