The original "110" model 9.8hp engine took a Champion J7J plug which is a short reach. An "L" long-reach plug is too long and the piston may hit the top of the plug.
In no case should a Perma-Gap plug be used in a conventional points-style (non-CDI) ignition, because that ign doesn't have enough energy to properly fire the surface-gap plug. It might work OK for a while, but will eventually foul out. Perma-Gaps only came in the long-reach anyway, so that's not even an option.
One equivalent plug to a J7J is an NGK B6S.
The original parts list also shows an AC M45 which I think may be still available as AC Delco M45. There are quite a few of those listed on eBay, as well as NOS J7J plugs.
I looked at the Champion website and the AC Delco M45 plug is listed as an equivalent to a Champion J8C. So, the M45 might be an alternate, hotter plug for prolonged idling.
The Champion plug has a copper core with a broader heat range, so maybe there's no other Champion equivalent to the J7J because the J8C covered the heat range well enough.
Looks like an AC Delco M44C (if you could find them) plug would be equivalent in heat range to the discontinued J7J.
The NGK B6S would be a good choice, old Mercs seem to run best on Champions and NGK plugs, and most auto parts stores will carry the B6S (and it'll be a lot cheaper than a marine store!).
Note
HTH...........ed
