When I had my '40 1 1/2 tonners they all had saddle tanks. I don't think anyone repops a direct replacement or anything even close. With the under seat tank removed its easier to mount a seat from a mini van as an added bonus. IMO
You might try a metal auxiliary gas tank for a boat. They come in various rectangular sizes. You would probably have to fab some type of frame for the seat to sit on since I doubt the boat tank would be be to support the seat. Some even have provisions for a gas gauge sending unit. Just a thought.
Bouncin', A friend of mine has a tank like that, I have spoken to him and he will have a look at it to check the condition. It is located near Birmingham in the UK.
Edit: He looked for it and it is no longer there. He thinks it must have gone during the last tidy up. Sorry, I thought I might be able to help.