I'd just replace the whole thing
In fact, I consider any toilet that sells for under $150 to be "disposable"...not worth the price of a rebuild kit (half or even more the price of a whole new toilet), much less any replacement hardware that costs more than $5. Although I push preventive maintenance, it makes more economic sense to use a cheap toilet till it starts to need work, then just replace it. The only time I have a problem with that is when somebody decides to toss out a $700-$900 Wilcox Skipper or Imperial because the rebuild kit for it costs more than a "disposable" toilet...that's kinda like dumping a Rolls Royce to buy a Yugo because a tune-up for the Rolls costs more than the Yugo.As for whether you should just replace your Jabsco with another one like it or switch to another make, it's up to you whether to stick with cheap and disposable, or spend about $100 for one that will keep on going for years. The Raritan PH II has long been considered the best manual marine toilet in its price range (under $500)...they seem to keep working even when neglected for years. I also like the Raritan Cricket, which costs about the same. Or you can spend $500-$600 for an electric toilet, but unless your boat is at least 30', I think that would be overkill...and while there are "conversion kits" that allow you to turn a manual toilet into an electric toilet for a couple hundred dollars less, I don't recommend 'em...'cuz about all they do is turn a decent manual toilet into a poor excuse for an electric toilet.Whatever head you decide on, just don't spend any more money on your toilet...replace it!