I have a bike from that time frame (a 1987 Diamondback) with similar brake setup If you can't find a disc fork, you can get better stopping power from your current brakes by replacing the pads with Koolstop Salmon. I used Supra 2 on the U-brake and Eagle 2 for the front cantilevers on my bike. Also shorten the transverse cable on th e front cantis for better leverage.