The only way to qualify this is to say I bought mine ('05 ZX-50) w/ 23,000km in 2008. It has gone thru four different incarnations. The dealer installed a primary gear set/Malossi variator/Contra spring/19mm delorto carb, when the bike was brand new.
I installed a malossi sport cylinder, Technigas RS II pipe, delorto 28mm carb and de restricted CDI. That lasted 3 years, and 20k miles of daily driving
Next life was a 50mm Jiang wayne cylinder, and engine case porting, w/ a 24mm OKO flat slide carb, and Jiangwayne expansion pipe. another 6k miles in a year.
As it is now... Everything has been changed to make it a mile speed bike, and it easily reached 82MPH without being fully tunned. I only have 32 miles on it since that build.
Point is the bike has gone thru hell, and looks/runs better than the day it was assembled in Taiwan. Still on the original engine cases, and rolling chassis.
I don't really believe this bike will ever be done. I have passed up several purchase offers because I will not sell it to anyone that does not realize what it is, and what it takes to keep it running. 20 years from now I hope my grand kid can show it off to his kids.