Actually Robin, if reducing the compression is what you are after, adding head shims is not the best way to go in terms of 2 cy performance if the head has a squish band, because it moves it away from the piston reducing the squish effect. Better method is to hog out the combustion chamber a little just above the piston. Carl Shoup introduced me to this trick and with LA engines it works great. I have a jig built to do it on a lathe but you can do it easily with a Dremel and burr.
Pics below show a before and after of an LA40 head.
Phil