If it is the 40 Lumen version shown here https://www.pinterest.com/pin/167407311118259929/ (top right) you are in real trouble trying to duplicate the lens. It is oval shaped and has both a clear and a focused portion. The 60 lumen version (top left) has a clear lens and while still odd shaped it would be easier to duplicate.

Maybe the best thing would be to go back to using a removable flashlight. Get one that focuses and uses the XML-T6 LED so you can light the road pretty well. The rechargeable 18650 Li-ion/throw-away AAA battery versions (works on either one) can be had for around $10.