Looks like you unthreaded it from the tail cap?
Can you unscrew that aluminum piece? It has to have a standard type switch inside?
Looks like a Thor type, to me anyways. They might just have a replacement on ebay or another style host rear cap might fit?
I just tried on ebay and got some 501B switches.. Its just on ebay you have to word it just right as I got switches for a Honda!!!
You should try because I believe they are out there, and someone should chime in soon..