I really don't know how to advise you -- but offer a couple of things to consider.
Is this a skeg hung rudder or a spade? Is the rudder stock solid bar or tube? If answers are spade and tube I'd be more concerned as the stresses would be greater on a spade and the tube weaker than an SS bar. Also, the severity and location of the corrosion would be important considerations. Rust on the quadrant, per se, may not be a big deal, but the rudder stock could be. Why don't you ask a yard what it would cost to rebuild the system?
I did a quick check on Contest 31s now on the market and they range from $45,000 (1982 in CT) to $4750 (1974 in SC). That's quite a range and may be either due to great differences in upkeep / condition, but also might be due to design and construction changes. I noticed that some of the 31's have skeg hung rudders and some have spades, so there obviously have been design changes over the years. You might want to investigate the design's history and manufacturer's reputation before deciding.