I've found that about 40K is the point where you might do some extra checking. They generally start to leak a little from the base and the valve guides can be starting to suffer from blow by. A compression test is going to be your best indication of where things are at. If it's really low or uneven it's off with the head/s and barrel/s, change the gaskets, belts, rings while you're there, check piston and rod clearance, and cleanup the valves and guides. Sounds like a lot but in general they are pretty easy bikes to work on. The only annoying bit is generally the rear pot gets hotter and can usually fail first, which is a bigger job to remove the head and barrel.
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