Nautical Canvas might be able to fix your seats. They're beside the Prop Shop in Kingston. Real nice people. I needed a piece of Sunbrella fabric for a umbrella repair. They gave me a large piece for free. Very impressed.
http://www.nauticalcanvaskingston.com/services/ When I was shopping at Tweed Sport and Lawn, Kirk had aftermarket seats.
The seats were the newer style and lots of different colour combinations. They were a much better price than the seats with a boat brand name on them. Maybe a better deal that fixing the old ones.
The base contruction looked to be the same as factory seats. Actually, they look very similar to the upgrade seat Alumacraft sells. Those seats take the 240 fine.
https://tweedsportandlawn.ca/Hope that helps.