Pivot tilts still exist in the smash games preceding S4, including Melee --> PM. Just tighter input windows and more precision, to the extent that it's not very commonly used because alternate options are available that are just as viable and easier to perform. But anyhow, pivot tilts are already in.
Can you not already perform a running side-b in PM? 5am so I may be pulling a dumb, but if I'm wrong and you can't actually just side-b out of running, then dash cancel would be good work around. Simply go from running --> tap down on the control stick --> input side-b. Crouching the quickest way to cancel dash into any standing action.
Both of these would save you he trouble of having to mod anything, as they basically exist already in vanilla PM (: