The Trustfire F20 "emitter swap" might be a nice first time "mod". Few threads here:
Original Post from old4570.
Newer post from keltex78.
Trustfire F20 found with correct 14500 compatible driver from Manafont and T-mart (US Warehouse). Your choice of emitter.
Makes a nice inexpensive Pocket Rocket (on 14500) which can still run AA's for convenience.
-Garry