I'm sort of the ain't broke, don't fix it kind of guy.
Four zone valves 34 years old. OK. So buy a couple (Honeywell seem good) and have them handy if something goes real bad.
If it was me, I'd be happy to be able to brag that zone valves lasted that long.
But who knows that a some point you don't decide to scrap that system for something newer and more efficient and that would be a fair time to retire those guys.
Until then and as long as they ARE working correctly, you sound OK to me.