I too have a Kracor retention system type holding tank
on my 1972 Irwin 32. Since Iam in the great lakes I will not be pumping overboard, but couldn't abide the recirculation type flush. I plugged the 3/4" recirculation fitting on tank and teed off the sink drain thru hull to the head pump
thru an anti-siphon loop. I am much happier and there is no reason to repace the tank. My holding tank
is a respectable 16 gallon capcity and doesn't leak.
Kracor holding tanks
are still available: check out their marine catalog-http://www.kracor.com/support/holdingtanks.pdf
Hope this helps,