Both 27" (630) and 700C (622) tires are available. Good value, with kevlar belts for flat-resistance, are Pasela Tourgard, available in both sizes and several widths. Crusty alloy, especially if it's not anodized, should clean up with Simichrome or paste wax that has some cleaner/polishing compound in it. I don't know anything about the Kotter frame, but if it's half decent, and you don't mind spending $100 - 150 on it, powdercoating would be the quickest way to rejuvenate it.