You can likely mount a new 1/4" jack in the bracket that clips to the strings. Indeed, a picture would help.
I strongly recommend that you not go with Radio Shack jacks and that you not use a stereo jack as suggested above. Here's why. Believe it or not, the dimensions of 1/4" plugs (tip-size, shape, etc.) are not really standardized. Take a look here
. Lower cost, lower quality jacks are usually not well-designed in terms of their tolerance for variations in plug-size. They often lead to intermittent connections on the bandstand. It's no fun. In addition, the Radio Shack products, especially their lower-end ones, tend to have flimsy contacts. They are simply not up to being repeatedly connected and disconnected. Understandably, they also don't tolerate abuse very well.
Using a stereo jack exacerbates the problems because there is even less room for error and, given the dimensional variances of 1/4" plugs, you can be asking for trouble.
The solution is fairly simple. Use a Switchcraft or a Neutrik 1/4" mono jack. They are built like tanks and make very secure connections. The $5 or so more you'll spend will help you to avoid huge amounts of frustration and aggravation.