The British Seagull I remember had a Villiers derived motor--so Villiers parts
might fit. These angines were also fitted to many British small motorcycles--the Dot scrambler and the James, to mention just two. If the thing is really hard to start it may be the capacitor or the magneto coil--sometimes the alnico magnets in the flywheel get a bit tired and need remagnetising. They would have to be one of the most trouble-free engines ever made, but that carby is not a great design.
I would also suspect a mud wasp may have built a nest in the water temperature regulating exit hole.The size of the hole is critical. If it is blocked the engine will boil itself dry and seize--if it is enlarged it will use excess fuel
. Fitted with the right propellor for the job they will push a barge. Ideal for a small multihull
auxiliary engine, just keep a spare plug
handy and premix the clean fuel
accurately. Check the bevel gear seals
and gearbox oil
and reverse shaft seal
now and again. Primitive but reliably simple--they were and still are used all over the world in the most remote
places, some are decades old and still whining along.