Ruger made the Bearcat as a convertible for a while, but recalled all the .22 Magnum cylinders because of excessive recoil and excessive strain to the frame. They were trying to tell us something.
Installing an S&W type sight would weaken a Bearcat frame to the point where it would be dangerous. I doubt you could find anyone willing to rebore/rerifle the barrel, either.
Your best bet might be to look around for an S&W Kit Gun in a .22 Magnum or in a convertible; they sometimes crop up on the used gun market, and are already equipped with adjustable sights. They are about the same weight and size as the Bearcat.
Jim