Mansfield Plumbing spun their entire marine toilets division off to SeaLand Technology in 1984...SeaLand discontinued the manual toilets (751 and 752) just a few years later to concentrate all their energies on the VacuFlush (which Mansfield introduced in 1978...so your Mansfield toilet is at least 35 years old. Mfrs only continue supporting equipment for a max of 10 years after they discontinue it. Which explains why you can't find any parts for it.
The Raritan PH SuperFlush is by far your best choice for a replacement. It uses the PHII pump which has been the top rated manual toilet for 30 years and Raritan has no intention of ever discontinuing it. Keep it well lubricated and put a rebuild kit in it about every 5-6 years and it'll easily give you 20+ of reliable trouble-free service. So it's definitely worth figuring out how to make it fit. However, I don't quite understand what you mean by "raise the base"...and until I do, I can't offer any suggestions about how to it.
However, there is a Plan B that just might work: the Raritan PHC "conversion:" And I
THINK sbo.com is the only retailer who still offers it. It's the compact version of the original PHII--same pump that Raritan is still using for the SuperFlush, but on a compact base and you'd keep your existing bowl (yes, it WILL fit the Raritan base), seat and lid. Check it out here:
Raritan LBA Upgrade for almost any toilet .
As for your second question, the answer is no. NO toilet, manual or electric, that's designed to use sea water should ever be connected to the fresh water plumbing and every toilet mfr specifically warns against it in their installation instructions. However, there is one safe way to supply fresh water to a sea water toilet: reroute the toilet flush water inlet line to tee into the head sink drain line (more on that after we succeed in finding a replacement toilet that'll fit your boat).
--Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein