You might want to consider a self-contained system--an "MSD" portapotty that's fitted for pumpout and designed to be permanently installed. A 5-6 gallon model holds 50-60 flushes...you'd need at least a 30 gal tank to hold that many from a manual marine
toilet. No plumbing
needed except a vent line and pumpout hose--so no new holes in the boat...and -0- maintenance
needed except for rinsing out the tank--which you can do with a bucket while it's being pumped out. And all for about $200--about what you'd spend just for a new tank or a new toilet.
The two brands you want to consider are Dometic/SeaLand and Thetford (I'm partial to Thetford). There are only two sizes--2-3 gallons and 5-6 gallons. "MSD" in the model # means it's designed to be permanently installed and pumped out ...no "MSD" means it has to be carried off the boat to empty it. The footprints are pretty much same for all of 'em...it's the height that determines capacity. However, "same footprint" doesn't necessarily mean that what's made today will fit your existing mounting brackets...in fact, the odds are pretty good that nothing will. Not a big deal to move 'em though.
Check out the Thetford 365 MSD... the 465 MSD even offers the "push button convenience" of an electric
flush, powered by 6 AA batteries.
The Dometic/SeaLand 711-M28 "Marine Traveler" is a VERY nice "souped up" MSD portapotty...Read all about it here: SeaLand 711-M28
It also costs a lot more than the more basic models, and also needs a lot more space...the tank footprint is about 20 x 20.
Hope this helps. If not, I'll be glad to help you sort out what will make you happy.