Have you tried this site
Thamesway Marine Products
Pits lane, Chedgrave, Norfolk, NR14 6NQ.
Landline: 01508 520 344
Mobile: 07852 520 206
Having the gearbox rebuilt in 1996-8(?) due to corrosion I had to visit for parts and found that his workshop is a cowshed at the bottom of his garden. All parts were available.
The only problem was on my return to Vancouver as the customs guy could not understand why I couldn’t have bought the parts in Canada!
Also today found all kinds of Thamesway ‘stuff’ on Google