Honestly I'd say just buy a new Maglite Led. The new ML125 3C model has an XP-G led, about as good as you can ask for in a Maglite form factor. Plus you'll probably spend more just converting yours than buying a new one, especially if you mess up considering you're not very experienced in building flashlights.
Or even better than Maglite, just get a dedicated high performance flashlight. One thing since you're fishing you'll probably want a water proof flashlight. A good one for fishing, would be a Solarforce M8, li-ion batteries and a much brighter XM-L led, people will think you're bringing a spotlight.