So long as your hydro system does not have some type of failure, a simple re-power would turn that old girl into a new machine again. Sticking with 18 hp a Briggs Vanguard would make a good choice!
The one thing that I picked up on is that you have not only kept up on the maintenance but per the manual. I know people that either don't do maintenance or they assume that they know what they are doing and make mistakes.
Yep, I've already have a Kohler Courage 20 hp on a Troybilt lawn tractor with 167 hrs on it, should be plug and play, will have to take alternator of the 18 hp and put on 20 hp, I've done same maintenance on the Troybilt that I do on the Gravley, I let a good friend use it when I got the Gravley, he is trustworthy with the engine so I'm sure it's still good, I mow the lawn every couple months to keep everything lubed in engine.