Welcome to SBO Forum.
Boat owners manuals from 1975 were not much more than and expanded sales brochure that told how to start the boat and put fluids in the auxiliary engine. At least that is what I found for my 1974 CAL35C.
Try looking doe a 1993 Ford F150 manual on the FORD WEBSITE.
https://www.fleet.ford.com/parts-service/resources/owner-manuals/
I had greater success looking for the manual of the auxiliary engine. I focused on specific systems not a "Whole Boat" one book fits all needs.
Design drawings of plumbing/electrical/installed systems... HA!.
With 20 years of Med sailing, you probably know all the basics. Your new to you boat has likely been modified by several previous owners. You would just be batting your head against a wall

trying to match the manual to what appears missing on your boat.
My best advice would be to take time in your boat to find out what is there and understand why it might be that way. I have been doing that for a couple of years. Some of the things I thought needed to be changed, turned out to be good ideas even if they were 40plus years old. Once I understood the old stuff, I only focused on projects that are compatible and improve the boats functions, leaving alone those that just work.
Good luck and fair winds.