I've been thinking about doing the same thing for my Mark I. The down side is I will likely have to have my receiver drilled and tapped. There are a couple of no drill mounts out there, but I'm not too wild about the configuration of them.
I have been told that because it is a .22, and the one piece design, you don't really need to get the high end holo sights. Less abuse from recoil.
I see a lot of sights that run more than the new price of a Mark III.
I may have mine machined in a couple months when I get a cash infusion. It'll run me about $65 including the Ruger mount. I'll be watching this thread with interest.