Compression test needs to have the throttle butterflys wide open and the carb slides held open. Also make sure the valve clearances are correct, allowing the valves to seat fully.
If those compression readings are correct, and considering it has 18000 miles, then its time for a top end re-build.
There's tons of postings on top-end re-builds on the web. Do some searching. You won't know what parts need replacing until you open it up. As a guess, parts and some labour costs (rebore/hone) might be $300 to $400. When you have the engine out of the frame, you'll want to inspect other things such as the clutch. Plan on replacing rings,front cam chain guide, cam chain, gaskets, oil seals, o-rings. If cylinders are worn excessively, you'll also need to buy new oversize pistons.
Some things you need:
Torque wrench
Valve spring compessor
Clutch holding tool
Micrometer
Feeler gauges
Vernier caliper
Valve lapping tool and compound
3bond 1104 gasket sealant
Lacquer thinner
You don't need a ring compressor, as the rings can be put on easily by hand.
If you get into it, the guys here will answer specific questions.