I don't know if any manufactures have ever produced a replacement fuel cell for an FL250. You do see them from time to time with box like aluminum cells in place of the factory units. All seem to be different and maybe made by owners... It's either that or a spun aluminum canister above.
I like the original design though. You can find them on eBay and restore them. I'm doing one right now.
Good luck and keep us posted.