By older Mini I'm going to assume it's still a Ranch model. You won't likely find a true 1913 rail for it, but you can buy a blank section of rail from Brownells and mill out the base for the integral ring bases. This is about the only time I would drill and tap a Mini receiver, after annealing the integral bases. That will get you going. Otherwise you'll need to modify a weaver style base to work with a file.