I had a 1954 United's Deluxe Bonus Gun. You shot hillbillies, whiskey bottles, scarecrows, and turkeys. Everyone friend and relative is still gripping at me because I sold it three years ago. I was planning on having my Chicago Coin Twin Rifle by now. So as far as the cool factor went and popularity of my guest, it ranked high.
United's are built like a tank. My guess, without seeing it, would be that you just had to take it all apart, clean it, lube some of it, and put it back together. Very easy. You just need time, a camera, some super lube, acetone, contact adjust tool, normal tools (needle nose, crescent wrench, 7/16, etc.), a drop light, and some beer for when you are finished.
I can help you if you like. I even have a draw full of United 1950's parts, but they should still be good. That's why I have a draw full, they hardly go bad.
Any pictures?
http://marvin3m.com/arcade/pirgun.htm