Our Debonair started out with an IO520 as well. When it was time to repower we moved up to the IO550. Unfortunately I didn't get to fly at much after the repower. It was a real screamer though. It was a mostly original 1966 that had to dual yoke retrofitted. The only major work we had done was having a bulkhead in the tail replaced. The prior owner was a doctor who like to get a little spirited with it. It did not have the external stiffners on the tail like the C33F though. I love those butter smooth controls!