Bernie , I think the best way would be the simple way 1.check for leaks 2. visually inspect the filter and guess if the filter has been changed in the last twenty years 3. check the there is a hydro fluid level drain it remove the back cover and look for pieces of metal or wear on visible parts.The whole pump and and transaxle seem very well put together.Mine, so far been trouble free.
The speed control linkage is a spot which usually needs some repair and or adjustment. Someone did a great write up on how to fix that. That job was fairly cheap to do with a small amount welding that can be farmed out.
Ken I have seen some cranks on ebay most understandable are from engines with broken connecting rods so you have to wonder about condition. My motor runs well So I thought if the machine work is under a couple hundred bucks it would be bearable. If not a used one would be the next Thing.Tonight I began to prepared garage to dissect the motor and extract the crank. the though of all those parts layed out on the bench makes me a little uneasy but here goes... I'll try to get some photos of the alleged progress, John