I flew an XL SE with YS 91 ACH for a couple of years. They are very smooth and quiet and use about half as much fuel as a 60. The down side was they don't make quite as much horsepower and struggle a little through consecutive maneuvers. The motors have a very specific red line and will come apart if you exceed that so a GV-1 was almost required.
Gordie

I still have the used conversion with a used Hatori exhaust but a brand new YS 91 ACH for sale if anyone wants to try a 4 stroke.