I have a 91 Australian model and have just replaced, piston rings, all kips valves, all bottom end seals and a new stator and ignition coils for just under $500au......the kips valves themselves were around $40 each and took about an hour to replace being my first time, I got the piston n ring kit for $99au. which wasa Wiseco 67mm big bore kit.......and Oh My God does this bike satisfy........obviously prices will vary where u are but just go to your local moty shop and order all parts required.
But as someone else said, the Kips valves not working WILL NOT stop the the bike from running, it just wont have too much grunt, first thing Id do is take the plug out n see if it has spark, if so....it is obviously a fuel/air/compression problem, which are all easily fixed..
Good luck mate.!