She doent look that dirty . I would look through the exhaust port and if that valve is caked on with carbon or if the bearings are gritty after a good lube I would clean it on the inside
, I pulled off the pipes, the E-Valves are very clean, in fact I can see on the right side that the port is still a bit sliver, on the left side the E-Valve is even cleaner but the port is a bit darker,, I do know the owner did send the engine in for repair at one time, one of the cams was replaced and the bearines too, and also the Cam Cover was replaced, I know because the old parts are in the box,, the owner did tell me it was down on power, his guess was that it had a burnt Valve, but I'm guessing not, I'm windering if it's simply loose Tapets
to add, on my engine the Needle Valves are closer to the crank case than on the engine in Hobbsy's link,,, still I wonder what engine is older/newer
Saito Jim