Part no longer available
I was finally able to get to my contact at Jabsco...that threaded cap is no longer available. So there are a couple of solutions, the best of which IMO would be to remove the toilet and tank and all the related plumbing and replace it with a 5-6 gal self-contained system...e.g. the "MSD" (which means it's designed to be permanently installed and is fitted for pumpout) portapotty. There are are several good reasons to do this:
1. a 6 gallon tank can only hold about a dozen flushes from a manual marine toilet..barely enough to last one person a whole weekend, two people overnight. A 5-6 gal. MSD portapotty holds 50-60 flushes...and you need a tank big enough to last a few days,' cuz you cannot legallly dump a holding tank ANYWHERE in the Chesapeake Bay...or in any US waters. You must be in open ocean at least 3 miles from nearest point on the whole US coastline to dump a tank.
2. There's little enough storage on a 25' boat without sacrifcing 25-35% of it to a tank and plumbing.
3. Manual marine toilets and tanks require a LOT of ongoing maintenance...portpotties require almost none.
Thetford and SeaLand make 5-6 gal MSD portapotties
That would be your best option IMO...
Another choice would be a new tank. Best source is Ronco Plastics
Ronco Plastics They make top quality plastic tanks, have more than 400 shapes and sizes, and install fittings in the sizes and locations specified by the customer. Cost would be about the same as the MSD portapotty...and you could go with a 10-15 gallon tank (barely large enough to be worth the effort).
Or, if you really want to be creative... The threaded cap for an 8" Beckson inspection port will fit your tank
http://www.beckson.com/screwout.html The right size hole saws, 3 UniSeals
UNISEAL (scroll through ALL the pages to see how it works and to get to prices), and 3 6" pieces of PVC pipe are all you need to re-create the cap and fittings on your tank.
The least expensive solution and definitely creative...but more trouble than it's worth to salvage a tank that only holds 6 gallons.