The exhaust valve will not have been opening properly lately, and because of that it will have been overheated. The valve head, and valve seat, may have become somewhat burned. However if you get the valve opening and closing correctly, this may correct itself (depending on how burned it is). Because the GCV engines are a bit of a pain to work on, and especially to lap the valves (because of the integral cylinder head), my plan has been to correct the valve movement problem then run it at light load for a period, performing compression tests occasionally. It is to be hoped that the valve is not badly burned, and after a while the compression pressure will become normal. If, however, the compression pressure gets lower rather than higher over time, it would mean that the valve is burned beyond the point of recovery without lapping or perhaps valve replacement. The engine would then have to be dismantled.
I do not think there is anything you can do to clean the valve while it is installed in the engine, which will have any measurable effect. If the valve seals immediately when you install the guide properly, the crud underneath the valve head will burn away quite quickly when the engine runs under load. If the valve takes a while to seal, so you have to run it under light load for quite a while to get it to bed in, the crud on the valve may stay there during this recovery process. It is of no importance anyway, it is just a symptom of the valve's restricted movement.
Hence I do not recommend trying to clean the underside of the valve head by putting carburetor cleaner in through the exhaust port. I do not think it will shift the crud, but much more importantly, the carb cleaner is going to get into the cylinder and interfere with lubrication.
