Before you ditch your trailrat, you might want to consider the possibility of building a replacement battery pack yourself for it. Myra-simon.com has a short, albeit dated, tutorial on building a water-bottle pack at http://www.myra-simon.com/myra/bike/bott-batt.html
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If you use 5 rechargeable D-cells, you would get around 6.5 hr runtimes if you get the quite expensive ($39.95/2 pack) MAHA D cells. I was thinking about how it might be possible and advisible to use a battery holder to hold the cells rather than soldering them together if the holder can fit within a water bottle.

Alternatively, http://www.batteryspace.com/index.as...S&Category=882
sells ready battery packs and chargers for less than $100. You might still need to do some electronics work in terms of swapping out the plug for the one that would fit the Niterider.