Nothing was modified on that Aero Commander.
Not quite - it had un-feathering accumulators added, so that he could whip the props into and out of feather for engine shut downs/engine starts.
If I remember the Feds pulled Hoover's medical a few years ago so he just kept flying on an Australian licence.
It was in the early '90s. Australia gave him a medical and licence, and he did one year of shows in Australia, before returning to the USA.