136psi is probably an absolute minimum, I would expect 160 - 190psi on a good motor. You did the test correctly, though I always hold the kill switch or ground the plug as it can damage the ignition system to kick it over without the plug instaled or grouded (the kill switch simply grounds the coil).
Top ends are easy, if you are worried about it, take it apart, mic everything, clean the pv's and put it back together.
FYI - you can have completely worn out rings and pistion and still have good compression. I went far too long on the top end on my 1993 KX250, including a solid 3 weeks of dune riding (8 hours a day, every 7 days a week:)). After all that, the ring gap was much bigger than any of my feeler gauges (Probably around 1/4"), the piston to cylinder cleanance was huge and it was still making 175psi of compression and running strong.