I have never had any luck heating out JB Weld. If you can that is awesome, but it has yet to work for me. I see three other options.
1. Carefully Dremel the retainer body out and glue in a replacement. My TAPs guided me through this on a 75mm mount, though instead of a new glue in retainer I switched to a thrust plate. Still, there was definitely enough room left on the motor mount tube to glue a new retainer on. I used a relatively narrow metal bit with cutting ability on its sides as well as dead on. I cut the retainer in five spots then jammed a straight screwdriver head between the retainer and tube, and pliers to finish removing each section.
2. Replace the aft end of the motor tube and add a second centering ring. I imagine you could do this first by using that same Dremel bit to cut a circle at half the material diameter of the centering ring, then a small cutting wheel inside the motor mount tube further forward than the aft centering ring. Butt up your new centering ring, cut a section of new motor mount tube to length, and epoxy it all into place being careful about alignment. Glue a new retainer onto the new tube after that epoxy is dry enough to stay in place.
3. Use a small thrust plate equivalent. Remove the retainer and possibly part of the motor mount tube, but then use an airframe width centering ring or custom cut wood part to mount the retainer. This would be a simple matter of buying parts, if the rocket was larger, but I do not know of any this small. No 29mm flanged retainers either but a stub of motor mount could be enough.
I bet you could tape motors in against that retainer stub, too; far less elegant but no additional work needed.