Measurements on the tank show it to be about 30 gals, which is in keeping with the factory specs.
I was just surprised to find about 18 inches of foam filling in a cavity in the tank coffin ahead of the tank. A much larger tank could have been installed but wasnt. I'm guessing the coffin mold for the 18 is the same as the 20, and they just put a smaller tank in because it was a "smaller" boat.

I think I'm going to go with the plastic Moeller tank that sidelock and sceptre20 refer to.
Holds slightly more gas than stock, fits in the coffin perfectly without any mods and is about half the cost of a fabricated aluminum tank.
I think it will suit my needs perfectly.

Like I said I think 47 was an option and would have filled it up, although blocking it to the front would help on weight distribution

Mine was replaced about 9 years ago. Don't know what size came out of it, but, it currently has a 35 gal. tank.