Replace the toilet...
It's at least 20 years old, and not worth putting ANY money into even if parts were still available for it--which they aren't. Mansfield was bought by SeaLand in 1984...SeaLand discontinued making manual toilets shortly afterward...no parts have been available for the 751 for at least 15 years...the 752 for about 10 years. Replacing a toilet is NOT that bad a job. All you have to do is remove the mounting lagbolts and disconnect the hosesc(close the seacocks first!)...take the toilet out. Fill the holes, drill new ones for the mounting bolts on the new toilet, screw 'em down, and reconnect the hoses. Shouldn't take you more than an hour.