The tank manufacturer on Karma, an 1981 ODay 34, is Florida Marine Tanks as Dave indicates above. They pulled the plans for the original tank and built me another one, using thicker metal and painted the tank for more corrosion protection. Cost for the 30 gallon tank was in the $300 range.
I would expect that you'll find that they also manufactured your's. If not I'm sure that Ruddy @ D&R can point you in the right direction.
The question is will you replace with the original capacity or reduce the capacity to insure a fresh fuel supply from season to season?