Given the ultra-low current draw of LEDs, have you considered making up a battery holder for 8 1.5V batteries (AAA would work fine) and just running them that way?
One thing you need to consider about LEDs is that they are light emitting DIODES. This means that current flows through them in only one direction. Since the sled produces AC power, the current reverses direction rapidly, and the lights are actually only on for every other little surge of current. This is actually bad for the lights.
Make sure you understand local operating laws in your area, too. It's -possible- that there could be restrictions about additional lighting on a snowmobile on the trails.