Just get a Centennial with its internal hammer; there is a longer barreled version too.
I've taken the hammer spur off a couple 3 J-guns for people, but it's probably better to take out the single action notch at the same time.
It's not too hard to cock a spurless revolver, but getting it uncocked while loaded is another matter.
The guns I worked on had been dropped on the spur hard enough to break it off, but the guns didn't fire.
Moon