Before doing ANYthing else, reset the valve clearances to specifications and check the compression again. You might be lucky, and the low compression is due to a tight valve clearance and nothing else. If proper compression cannot be achieved then here is what you are in for.
First, the cylinder head has to come off. I suspect that this requires the engine out of the bike (and life will be a whole lot easier doing the following work on the bench, even if the head can be done with the engine in-bike). But it's an air-cooled twin. Engine removal should be simple.
If the real problem is a burned valve, then at a minimum, the valve has to be replaced, the valve guide needs to be inspected for wear and repaired/replaced as necessary, the valve seat has to be re-cut and lapped with the new valve then cleaned up, and the valve stem seal has to be replaced. Then put the engine back together with a new head gasket. This is an air-cooled engine with two valves per cylinder ... this job is pretty close to as simple as it gets. But special tools and know-how are needed in order to repair the valve seats.
Once the engine is back together THEN look at the carbs, and it sounds like they need to be re-jetted.
Most common causes of burned valves:
Neglected (out of spec and ignored) valve clearance.
Overheating - air-cooled engines are susceptible to this.
Carbon build-up due to excessive short-trip driving (engine never warms up), excessive use of the choke, excessive rich mixture due to carb issues.
Carbon build-up because the engine is burning oil because of other problems (rings, valve stem seals, crankcase venting issues).
Carburetion set too lean near full-load. Lean idle and cruise won't hurt it (and helps to remove carbon build-up) but lean near full load is bad news.
Wrong oil, neglected oil changes, etc leading to valve stem and valve guide issues.
