Find a leather supplier, get a small bottle of leather dye and re-dye the entire sporran or whatever is plain leather. Then get some saddle soap and rub it into the worn area a few times over a couple of days. Once that's set in, rub out EVERYTHING that you've dyed with a good clean cloth and rub it hard. Then BRUSH on regular shoe polish and let the wax polish dry and harden before you buff it, wax polish it again and brush it once it's completely dry and then rub it down with a clean cloth. It won't transfer any dye or polish to your tartan. I do this all the time.

Chris.