Those engines formed, along with the Big Suzook GS's, the bedrock of drag racing for 20 years!
A HUGE support / aftermarket culture for them.
Any issues those bikes may have had are long since had fixes made available.
As for a "restorable" bike? I'm less sure on that. Some particular models are more desirable, of course. The original 900, sure; the Z-1R's, sure. A plain KZ1000? Probably not a real money-maker. But a good choice for a fixer-upper or 'restified' bike? Absolutely.
Just my two cents, and, as always, your mileage may vary.
Kirk