The late K3-on models got a different (one-piece) brake pedal that didn't need the spacer. Those are the most plentiful used parts, these days. The spring is a clean & simple alternative, it's the easiest for sure. Having a spacer made from 30mm aluminum rod, or thick-walled nylon/Delrin tubing (with the correct ID) isn't difficult. It's more effort and bucks than the spring, but not much. A machine shop could turn one, to your specs, in a few minutes for ~$35...if you want a solid spacer and can't find an original.
FYI...you are far better off running stock 4.00-10 Trailwings on a stock bike. It not only looks better, it also gives you stock gearing (which is taller than what you had with the shorter tire) and these engines are finicky when it comes to gearing. Shorter-than-stock gearing does nothing good for a stock 4-speed...just adds more revs per mile than are necessary.