You'll be spending $100 including tax and shipping to rebuild a 38 yr old toilet that Groco will tell you has a working lifespan of 9-11 years...better than the 5-7 yr lifespan of most compacts, but still obsolete and 30 yrs past its prime. Rebuilding the pump will not restore it to factory performance because parts that aren't in rebuild kits also wear...and if they're worn enough to need replacing too, you'll be out of luck if you need aany that aren't also compatible with the HF.
A '55 Chevy was a really cool car in its day...but if you found one in someone's barn, would you seriously consider trying to restore it--not as a classic museum piece, but as your primary transportation? 'Cuz that's essentially what you you'd be doing if you try to resurrect a toilet that's been obsolete for 20+ years.
Spend the money to upgrade it to an HF, or replace it with a current model of any compact toilet--even a Jabsco if there isn't room in your budget for anything better...it'll last a couple of years and is about the same price as the HC rebuild kit....and it's not a big a deal at all to drill a couple of new holes and fill a couple a new toilet won't fit.
Or you can play Russian roulette with the only toilet on your boat and pray that a) there's only one loaded chamber, and b) it's not a guest--or worse yet, your wife--who'll pull the trigger on it.