Does your Ruger have the tiny little screws/holes in the top of the receiver tube? If so, you should be able to just buy the scope base from Ruger, or any other manufacturer that makes a base that fits the Ruger factory hole pattern.
No holes, it gets a little harder. You can still buy the base from Ruger, but you'll have to take it to a gunsmith and have them drill and tap the holes to match the base. I'd guess $50-$60 bucks, minimum.