Hi Pete,
First things first; 964's don't have top ends, they have laterally displaced asynchronous side ends. So, I wonder if those who have discussed top end rebuilds with you are in fact referring to work required on the owner's top end. (I note they also refer you to a fluid control problem. Well, we should all be able to forgive a little incontinence; shouldn't wee?)
The inference that 964 owners have somehow been bypassed in the human evolutionary stakes is scurrilous. Personally, I have seen plenty of examples where owners are well suited to their 964.
HTH
Kind regards
Bob
I'm going for a rest; I may be gone awhile...