Hey old CB I advanced the timing on several 2100s. If I remember correctly there were 18 cooling fins on the flywheel. If you do the math it is 20 degrees for each fin. I would mark a fin ATC with a sharpie. If I wanted to advance the timing 10 degrees, I would shoot for 1/2 the distance between fins. The key is filed a little and then checked for the desired advance. Remember the flywheel is held on by the taper. The key only sets initial place and the taper holds it where you want, (get that nut tight).
In case this is still not clear: the magnet flying by the coil makes the spark. If it flies by earlier then the ignition is advanced. If you advance it to much, the saw will want to run backwards. Advance at your own risk. Rather violent kickback can occur on some saws. BTW the 2100 has a slotted coil so some advance can be given by that method. I also lengthened the slot on some of them.
I see your next question coming. What is a good amount of advance in a saw? Well 10 degrees is plenty in big and hot saws but you have to experiment. Each saw is different and some don't like advance at all. As in most things on this forum, nothing is written in stone. That is why it is call 2 stroke theory. Do your own trial and error. Nobody can do it for you. Mike