I'd keep the portapotty on a boat that size
Replacing it with a marine toilet will require installing a thru-hull/seacock for the flush water intake and a holding tank under a settee that--plus the hoses from toilet the tank and from the tank to the pumpout fitting--will occupy a good portion of your already limited storage space. So I'd stick with a self-contained system.If it's a small one that has to be carried off the boat to empty it, that's easy to solve...replace it it with a larger one that can be permanently installed and fitted for pumpout. The footprints are the same for all sizes...only the height changes. The "pick of the litter" is the SeaLand 965MSD...a 5+ gallon model. No new plumbing required except a vent line and pumpout hose that goes to a deck fitting. And because portapotties use so little flush water compared to a marine toilet, you'd need at least a 15-18 gallon tank to hold as many flushes as a 5-6 gal. portapotty holds. Staying with a self-contained system also means you'd have none of the maintenance or odor issues associated with a marine toilet and tank. Pouring a bucket of water into it while pumping it out is all the maintenance it'll ever need.