Here is a LED headlight I made using a 1-inch PVC pipe coupler, Luxeon 1-watt star LED, and a matching elliptical lens and holder.
I made my own heat sink from some scrap copper sheet metal cut round to just fit inside the plastic pipe coupler that is mounted to the lens holder sandwiching the LED between them.
I used a 1-watt LED as I am using it with a dynamo so I had to keep the power requirements below 500mA.
Here is the link where I got my LED and matching optics.
If you don’t mind batteries you might consider using a flashlight LED drop in lamp assembly like this.
I don’t think it will fit in your current halogen bulb housing but it would be very easy to put it in a PVC pipe or a PVC pipe coupler. Just go to a hardware store and find something it will fit in and mount that assembly inside. Here is the link where you can purchase this assembly.
For about $10 you get the reflector, LED, and driver circuit board. This is a very bright LED light! It is rated at 1000 LUX. It operates between 3.6 and 9 volts DC so your 6-volt batteries will be fine for it.
Be sure and look at the other DIY flashlight stuff as well.