The pushrod balljoint sockets in the rockers may be looking a little ratty. I can't see them very well in the pictures, but the exhaust one might be looking worn on the side of the socket closest to the center pivot. On cars, I've seen at least one pushrod projecting right through that socket - the lubrication there is fairly poor on horizontal crankshaft, vertical cylinder engines. Also though, I seem to recall that failure occurred when someone had become unusually ambitious with the valve springs and cam contours he'd used. Most people use thick, forged aluminium rockers when they attempt that kind of thing.
How do the ball parts of the rockers' center pivots look?
In these pictures, the bottom surface of the exhaust tappet looks as if it has a black area around the outside: is that the part that the decompressor pin contacts? Can you feel any depression there?
