Top end and pv questions 04 300

[nmhinton]

Hi guys,
my 04 ec300 quit on me the other day. The first sign was that the top end power went, then it just quit altogether. After checking the carb, I started to dismantle the top end. When removing the pv cover on the right side of the cylinder, I noticed that the screw holding the actuator plate for the pv was almost entirely unthreaded such that the rod had no effect on the valve position. Is this reason enough for the bike not to run?
I'm not too familiar with how it works. Anyway, I took the cyclinder off everything looks decent. I am thinking of doing a new piston/rings. To my knowledge, the bike has never had a top end done, and seems to have been ridden quite a bit. What are my options for kits? Will a caliper measurement tell me if I need to go to a "B" size piston, or do I stick with "A?" Where can I find info on tolerances?
Also, I want to clean up the pv. Is it best to remove the entire assembly, or can I just spray it out with carb cleaner? Thanks for your replies.