There are two Moeller brand plastic tanks available that are ~23 gallons each and were/are used in new C-Dory 22 Cruisers. I see that you want more than 46 gallons, but these do fit into the usual location beneath the splashwell. I found the tank information in the (fantastic) archives here, and then made some phone calls to inquire as to availability, because I'm considering replacing my tanks.
The part numbers are as follows:
Moeller FT 2339 = starboard tank
Moeller FT 2340 = port tank
I spoke with Moeller and they confirmed that these are current part numbers and that they own the rights to make the tanks. They said that the way to order them is to go through a Moeller dealer (as opposed to ordering directly from Moeller). There seem to be quite a few dealers, including large places such as Fisheries Supply. I got the impression that they are a non-custom tank but that they don't typically have them just lying around but rather make them as necessary. I also spoke with the folks at Triton and they will sell them directly as parts.
I haven't yet decided whether to get new tanks at all (I have the 20-gallon "white" plastic tanks), but if I do I would choose these Moeller tanks for an "easy" installation. (Saddle tanks seem like they might be a good option but would be more involved as they'd need to be designed and fabricated, etc.)
I have sketches of the tanks (also found here in the archives) which I'll see if I can add here - since I can't remember whose album I found them in. I believe they would fit in any 1987 or newer 22, but of course you'd want to make sure.
Sunbeam :hot
Here are the drawings: