I put a mineral glass lens in mine and it's pretty nice. Not as impact resistant but won't scratch or collect dust as easy. Litfuse had some
HERE . Otherwise post in the BST forum for a stock lens someone has replaced already. Mine was pretty beat up when I replaced it.
You need to remove tail cap and battery before heating up the light to loosen the head. Then a small needle-nose pliers or snap-ring tool to unscrew the reflector. Some boil the unit for a few minutes (double bagged in Zip-Locs), some use a heat gun, others have gotten lucky with brute force. Mine was very tight and took a couple of tries (boiling) and clamping it in a plastic jawed vise with a strap wrench. Was well worth it. Look into goldserves dual brightness mod
HERE. It also is worth the time!