Seems popular mowers nowadays have a 10" bolt circle, punched out for single and V twin engines, if large enough for a V twin. The electrical connections and starters are such that one wiring harness fits both. The fuel line fits both. The cowling fits both. The only difference is the choke and throttle cables are too short going from single to twin since the carb on the single is on the left side and the twin is around in front. The pulleys for the dual belts are interchangeable, as are the belt lengths. While I was in there I just put new belts on.
I did two in the last couple of years, single to v twin, and it was a piece of cake making the changeover. The choke/throttle problem was easily solved with ebay parts. Some mount where your OEM mounted and some (the ones I liked) didn't so I just attached them to the side of the plastic shroud that goes around the operator's station.