My humble opinion on heads: install new
When I bought my 1980 Marine Trader 41 in 2007, the holding tank and associated hoses had not been used in some time. The previous owner disconnected them, left them in place and discharged to sea. I threw out the head, all the hoses, and the holding tank. I bought a brand new Sealand Dominec system. This consisted of a manual jabsco head, holding tank with macerator pump and a tank watch system and sanitary hoses. At the time I think it was around $1500 . I did all the work myself as it wasn't that complicated. I use an RV organic waste additive to digest the effluent in the tank, and then flush the system with fresh water before going back to the dock. It has given me 7 trouble free years. Having to deal with a previous owners old head and holding tank, especially considering the small cost, doesn't make any sense at all.