Up until recent times, it was common for soloists to improvise or compose new cadenzas. Somehow, with the stratification and separation of roles (composer versus performer, classical player vs. improvisor), this approach has been neglected. Such"tampering" is considered off limits, even sacrilegious.
Yet, the music of the past is not a sacred relic, but a living organism. In that spirit, I've composed a new cadenza for the Mozart G Major violin concerto.