Pry the cover off with a flat blade screwdriver; with the rivets weakened by drilling they will snap and the cover will come free.

Step up to a slightly bigger drill bit and drill out more of the rivets from the gear housing as shown, and from the rivet holes in the cover. The goal is to remove most of the rivet material from the four holes in the housing, and all of the material from the holes in the cover.

Repeat this drilling until you reach a 1/8" drill bit. This is the largest bit you'll want to use.