Mike, we had to make a similar decision when we bought Miss Fidget three years ago, as her outboard had been stolen a little earlier. We had the advantage of being able to look at two-stroke engines, and chose a Tohatsu 9.8HP. It's close to the limit of the size that will fit, and the mounting bracket made by the boatyard to replace the one cut off by the thief wasn't initially the right size, so that the depth of the propeller in the water was wrong, and the performance was badly affected. We didn't feel we had it right until the second year we owned her. However, we're now very pleased - it's not excessively noisy (at least, not for a two stroke outboard!) and she drives forward very well. In calm waters, such as in a marina, she also manages pretty well in reverse at slow revs. There's quite a bit of oil smoke on starting up, but the exhaust is vented through the propeller and we don't have any problems of smoke in the cockpit once we've got going. However, we're not able to get the closure plate into place to fill the outboard well when the engine is up, so, although we usually raise the engine while we're sailing, we still have a fair bit of turbulence in the well.
I believe that the early Seals may have had smaller outboard wells than the later ones - but I don't know whether Miss Fidget, no. 69, counts as an early or later model in this respect.
There's an article in the Super Seal Technical Handbook (ref 82/4), written by B. Campbell-James, that is available from PSSA that describes the use of a Chrysler Sailor longshaft outboard on Siola, no. 27. He seems to have been very pleased with the eventual outcome, although it took a bit of time to get the details right.
We're based at Wicormarine, between Fareham and Portchester, and you're welcome to look at Miss Fidget, if that would be useful and convenient.
Gilliane
Miss Fidget, Super Seal 26, no. 69
Delphine, Parker 275, no. 41