i once used a universal spark plug wire kit for a car. Wires were cut to whatever length you need and the kit supplied rubber booted connectors for each end. The boots had a crimp connection so you just cut and strip a wire, bend the wire over the cable, and crimp on the plug boot.
Go to the auto parts store and see what's available.. They probably have single boot repair kits.
Shortening the cable won't improve performance but it sounds like a ignition tune up will.