When I got my used boat, which came with all the original documents, there was no "user manual" for the boat.
The only document which carried the Bavaria logo was the electric system schematics. Apart from that there were simply the OEM manuals of all the installed equipment - mast, furling, engine, chargers, pumps, electronic equipment, winches etc.
But even with this extensive documentation, when recently a part broke in the throttle, I had to search the net and find the relevant drawings to help me with fixing it.
With a 30years old boat there is a good chance that a lot of components were replaced and not necessarily same as originals.
So the best thing would probably to start identifying all installed equipment and search the net for manuals