The compression should be 185 to 200 psi but that is only an indication as to the health of the top end. The only way to really know if anything needs to be replaced is to tear it down and measure things.
Usually if the compression drops by 10%, you are passed the wear limits but again, it's only an indication and only measuring the parts will let you know for sure. I like to measure the compression after doing a complete top end as a base line and then check it after 20 hours or so depending on how hard I'm riding.
I'd suggest getting a manual if you plan on doing your own maintenance.