The most important question: Are you a racer or a casual rider? What will make you happy.
If the valve is simply hard face worn yet the seat is in good condition, just replace the worn valve. It will run fine and mate itself with breakin running. If you must have top precision to be happy and comfortable, open your wallet and have them all replaced and machined/cut.
With those questions you will get a lot of opinions that are skewed by whatever people spent their money on and they try to justify their decision by propagating it.
Imho. Just go oem genuine Kawi valves for replacements at 80-100$ each and ride on. If you opt for changing to some other aftermarket valves and springs then you will have to go whole hog and send the head out and spend the $1000.
For piston and rings, again imho go oem. The small amount of power that a higher compression piston will give does not justify the strains on the rest of the system, the change in the ignition timing, the higher octane gas required, and has no benefit to your riding description needs.
If you want a reliable bike at modest cost, learn to inspect well; replace only what's necessary with new oem parts, torque to specs, keep it clean and ride it often.
As example; have a well worn '05 here that is rock solid and just keeps going. But we do stay on top of basic maintenance. Recently replaced the left intake valve and piston rings only. It is the best bike in the fleet for the weekend rides and just going for a rip.
Now, if you are a wannabe racer vying for front lines, different story. Advice is still go oem but replace whole "sets" and precision recuts at a time.
If the seat is in place, no evidence of movement, uniform wear all way around with no obvious chips or deformations, it is fine. For valves, look for "cupping" around the rim. Cupped valves eventually will pull up and stick in the hole, promptly bending the stem, and immediately there after snap off and drop - causing carnage. Here are some valve pictures. Hard to see the wear. The valve on the left is good. The valve on the right is cupped and was replaced.
Ok great. I've heard of cupping. I see it. It is very hard to see. Feel better about the seats to now. Not sure what I'll find when I get in there but I feel alot better now brother thank you!!
You are welcome. Happy to help. When you get into it, any questions just post up some pictures and folks here will certainly help you out.