Here's a link that you may find interesting - GRAMPIAN 26
The boat you are looking at is hull #899 - name is Albatross - was owned by Mike Stiles.
Ditch the old head and holding tank, and put in a fixed Thetford 365 MSD. You can use the existing pump out fittings, and existing air vent for this unit. You'll get about 63 flushes from the Thetford, versus 20 flushes max. from the old head and holding tank.
The rudder shaft is solid s/s, and I really doubt that will ever be a problem. Make sure though that you remove the rudder every year when you go on the hard. Take the rudder home and store it upside down in an area that doesn't freeze.
Removing the rudder is very simple, and you may wish to do this before launching so you can verify the unit is solid and doesn't need to be glassed around the edges etc. You must take every precaution not to get water into the rudder core.
Check the bulkheads carefully. I'm almost positive that there will be some rot in them where the chainplates are fastened. Replacing the bulkheads is an easy job, and you will probably just find the bulkheads between the head and v-berth need replacing.
Shoot me a note if I can be of any help. We have had our G-26 since 1999 and absolutely love her.