You have a compact manual marine toilet. The brand is Groco...they make pretty decent equipment. The model (HE) is obsolete, however...updated to HF in 1985. There are no kits or parts still available for the HE...so you'd have to update it to the HF to replace any parts in it..and from the look of it, it's in serious need of a complete rebuild kit.
Your holding tank is totally separate from it.
All that said,... a marine toilet plus a separate holding tank PLUS all the related plumbing cannot leave enough storage space for a ham sandwich on a 22' boat! It's not worth the cost to make it a functioning system. However there is a solution that actually makes sense on your boat: a portpotty, which you can get as a portable version (carry the tank off the boat to empty it) or the MSD version, which has fittings to plumb it for pumpout. All you'd need is a pumpout line and vent line...you already have everything else.
A 5-6 gallon model is household height and holds 50-60 flushes...you'd need at least a 30 gal tank to hold that many from a manual marine toilet. 0- maintenance needed except for rinsing out the tank--which you can do with a bucket while it's being pumped out. Total cost including the pumpout hose and vent line is about $300 (you already have everything else)--a fraction of what you'd spend for everything you'd need to make your existing system workable. And the best part is, you have all the advantages of a toilet and holding tank without giving up a single square foot of storage space.
Check out the Dometic 975 MSD Sanipottie This retailer (Defender) has it for $200. Also has the best prices for hose. And I'll be glad to help you sort out the details of installing it. The first step: remove the existing toilet, holding tank and ALL the related plumbing. 2nd step: a very thorough cleaning of the boat's interior and bilges. Then you'll be ready to install the new self-contained system.
--Peggie
