If you find out the diameter of the fiber optic rod used, then the following steps are all you need to fix it,
Cut a piece of fiber optic slightly longer than the sight.
Heat the end towards the shooter, and then pull towards the front to make a bulb.
Heat the part of the fiber optic at the front of the sight.
Here is a sub <2 minute video from Brownells on youtube showing the process:
Quick Tip: How to install a fiber optic rod in front sight - YouTube