Have you considered a dampened counter shaft sprocket as a cheaper (slightly less effective?
) alternative? The splines on your counter shaft are the same as those on a 690e/r/smc which use a rubber dampened sprocket. The difference being that the 690 sprocket is slightly thicker which could be easily cut down to fit the exc. Sprockets are available in 15/16/17 tooth for around $30.
I have not personally tried this but I have put a non-dampened exc sprocket on my 690 using a small spacer behind it, so it would make sense that it could be done the other way by removing material from the inside of the sprocket.