I assume your Single-Six has adjustable sights. If that is correct, you could replace the rear sight with a Bowen Target rear sight at a cost of about $90.00. If you want a Patridge front sight instead of a ramp, you can use the Ruger Patridge front sight for a Ruger Mk. III Target, which is available from the Prescott factory for less than $10.00.
If your gun has fixed sights, it can neither be readily nor inexpensively fitted with adjustable ones, nor with optics. The options are to sell it or be satisfied with what you have.