This stuff might be another option. I used it to build a plexiglass enclosure/mounting platform for some audio equipment, and the stuff is still holding after three years.

SureHold® Plastic Surgery - Ace Hardware

http://www.acehardware.com/product/i...ductId=1390216

I know a big concern for our headlights would be whether it ended up drying clear, and it says it does in the product description, but I personally think that depends on the specific formulation of plastic of the item. It worked well and dried clear on the plexiglass (dark smoked) I used, but no guarantee. If nothing else, it would be a step towards preventing the crack from getting larger. If it dried perfectly clear, even better.