Or lap the thrust face of the cam gear, but that's already been suggested.
If you decide to change the dimensions of anything, first use a straightedge to check the alignment of the cam gear to the crank gear. Then see if moving the gear forward (using shim stock) or moving it rearward (lapping the cam gear thrust face or lapping the retainer plate) brings the alignment of the two gears closer to perfect.
And yes, I know there's some alignment leeway in the width of the chain vs. the widths of the gears. But if you're going to do anything that will affect the alignment, do it to make it better not worse.