Hi Fenders
Your engine should be the MD2020-D which was the last series manufactured under said name.
The saildrive could be either the 120S-E or the 130S-A as they were both used with your engine.
What Symphony says about corrosion is correct and apart the checks he has suggested there is not much you can do.
By design the saildrive should be electrically isolated from the engine and the ground of the boat electrical system, hence a galvanic isolator may not produce much effect.
When I purchased my boat 3 years back I have found a Kiwiprop feathering prop installed and it is made by a DuPont composite material and a Stainless steel hub.
You may read about at the following link
http://www.kiwiprops.co.nz/cms/index.phpI realise it may not be the most performing prop when compared with Autoprop or Flexofold, but in turns it is not affected by corrosion.
However Personally I am happy with the performance, about 7kn @ 2600 rpm (MD2030-35 foot boat 5.5 tons displacement flat sea- no wind).
Being a 3 blades it has a bit more thrust, while being feathering it performs very well in reverse and it has less drag than your prop when sailing.
Look at the picture, It is about 10 years old now.
Only draw back .... about 1000 pounds + VAT.
Cheers
Oliver