I looked at this recently and decided there is no easy way to modify the stock parts. The center web on the cam gear has a wave in it for rigidity. This means any modification such as cutting up two gears and fixing them back together with a plate won't do it because there isn't enough material left to work with. If the center web was flat then I think it would be fairly easy.
To retard my timing 4 degrees I cut a new keyway offset 178 (or 182 depending on how you look at it) degrees from the original and remarked the timing dot. Worked great but as pointed out the keyway itself is the easy part of the job compared to pulling the cam and shrinking fitting the gear.
