The gears I ordered from Germany have arrived. They were a perfect fit, as anticipated.
I replaced the gear in the donor locomotive and now have two Minitrix U28C locomotives, one in Southern Pacific and one in ACL livery, in perfect working order. I also have two spare gears for the Minitrix U28C/U30CG/U60CG chassis.
And this should conclude the Cotton Brute saga. Thanks for following along.