Spring option: You will need to modify the tail spring to fit a pair of 26650s in a 2D cell Maglite, so purchase of the shorty spring is recommended.
Magnets option: These are used to make contact between the top of a 26650 battery and the base of the Maglite switch. Most cells are magnetic on the + end however not all are. If yours are not and alternative is to place the magnets (typically 2pc) on the top and put a dab of epoxy around the edge of them to hold in place.
Lens option: Many dropin bulbs including some of A.S.Flashlights will work fine with only the factory lens. The high temp lens is more transparent so output will be greater with this. If your using the 3x XHP-50 bulb its highly recommended that you also use the high temp lens.