You need to find those few Super Seals that used to have an outboard well and in the process of converting to diesel filled it in. It is just possible that one of them kept the old plug. I don't know who would have the records of which Super Seals have done this. I do know that Walter Brown (Ard Righ) did. You might ask him. As he was a previous Commodore he may well remember who else did.
I'd be surprised if Parkers kept any as spares.
I converted to diesel but kept the old well and plug intact ... glad I did. I can get to the prop from the cockpit ... and have had to!
Chris. SS26 103. Elsa.