Don’t you need more than just the voltage though?
Not to figure out the rough parts you need no, when it comes to specifics yes you will need to know the Ah rating of the battery as well, this is so that you can figure out the type of driver(control board)you will need to power your light.
Once you know how the control board works you will be able to calculate the run time of the light.
[edit] as JAG said this is reliant on you getting the bits from Troutie who will be able to advise exactly what is possible [/edit]
Just re-read your bit about not wanting to shop online this could be a problem for your light upgrade. Most of these parts will not be available at the likes of your local electrical shop. Troutie gets parts from all over the world for his lights, removing the shop online option it will make it almost impossible to get the stuff you need.
I have been racking my brains for lights you can buy from a shop and can only think of the exposure and hope lights both of which are pretty pricy.
If you decide to buy online and want an entire new light, Smudge from the forum has an online shop specialising in mountain bike batteries and lights. Below is a link to his shop
http://www.mtbbatteries.co.uk/our-products/