A 20 gallon plastic tank was an option for later CJs. I wouldn't trade mine for anything. Big and will never rust.
How is it that you have no skidplate anymore? I only ask to suggest that if its off already, now is a good time to drop the tank, flush it, and replace the in-tank fuel filter/intake screen. My tank had sludge like you read about and the filter was black.
Anyway, check with used parts places for a replacement, or look to the aftermarket, just be sure it is the right skidplate for the oversized tank.
Also, you or a fabricator could make a pretty beefy one up to meet your needs. It's not complicated and only a matter of taking good measurements, using good steel, and making good welds.
IMHO keep the 20 gallon tank.
'82 Scrambler - NV4500/4.5" lift/33" Toyo MTs/4.0/twin stick, and my windshield washers work! Full 1/2 cab Cage
2004 Grand Limited 4.7 HO - OH Boy!
2010 Grand Laredo 5.7 Hemi - SICK - work vehicle -gone - replaced by an 08 Tahoe - nice ride.
"Both my Doberman and my Jeep can accurately be described as 'black and rust.'"
"I got a fever . . . and the only prescription is more cowbell"