The following needs a sufficiently long loop to work, and would be a lot easier to explain with a picture but I haven't sorted hosting yet so here goes anyway...
If the loop is long enough - pass the loop through the end of the new buckle, such that the buckle is free to slide along the loop of webbing. Now, to illustrate the principle, stick your finger or something like say a biro through the end of the loop such that the buckle can't slide back off the loop and is now trapped in place.
Then you simply need to replace the finger/pen. I suggest a maillon rapide, available from all good climbing shops. I normally get mine from hitchnhike. A
Maillon Rapide stainless long should do the trick. The exact dimensions are on the maillon site, so it's normally not a problem selecting the right size.
Once attached the screw part of the maillon should be under the webbing loop and the finished bodge should look quite tidy, and be fairly serviceable if it's a snug fit.
An alternative ugly fudge to get the bag back in use whilst waiting for the maillon to arrive is to again pass the loop through the buckle, as described above, but then pass the buckle back through the loop as if tying a lark's head knot (or cow hitch if you prefer). Ugly, but should hold, assuming the loop is long enough.
And if you find yourself in need of a device for lowering yourself down a lift shaft then a maillon will hold a lot better than anything John McClane normally has at his disposal