O.V.P. is just a Zener Diode, relay and a fuse or two - depending on variation.
You are really better off buying a new one - they're relatively cheap.

F.P.R.'s E.C.U.'s etc are all subject to extremes of heat and cold plus dampness. Any wonder that they start to misbehave after twenty years plus.

You average consumer computer electronic product would have died long, long, long ago id subject to the same ambient conditions.