Saving Old Seagulls

My 102 starts every time (cold or hot) minus the prop. in a tank, and runs smoothly at all speeds. In the water (I have no clutch) it will start easily on almost full throttle, but runs, misfires, spits out the carby (no choke either) etc, and then after 30 seconds or so settles down to smooth runni...

There was a Seagull of the type you described on display in the Fremantle Maritime Museum, Western Australia. It had a brass exhaust. It is now in storage.
You could contact the museum ... freorcpt@museum.wa.gov.au

I checked the spark plugs from my 102, C100 and 40+, each plug was 22mm across the flat.

What size tube and socket will fit the nut size of the Seagull spark plug?

I only have 2, but use them both, at the same time!