If you're talking about converting a mains powered incandescent lamp to battery powered LEDs, you will either have a considerable drop in lighting, or will need a substantial battery and charger to get a similar light level for any length of time.
Assuming it's a directional light with a 60W bulb, you will want at least 6W and if it's not directional probably 11w which is quite a load for a low voltage battery system.
If battery power is not important then buy a mains powered ready made bulb.
But if you do want to go battery powered, Your best bet is as John U suggests, buying ready made 12V bulbs; either car bulbs or GU10/Mr16 12v spotlight bulbs and using a sealed lead acid battery and charger. If you've got the space A 2.3Ah 12v battery should give around 4.5 hours with a 6V bulb.
Be warned though: the charger will take the battery Voltage to over 14V so you either need a bulb that will take that (car ones should, but not many cheaper ones state the upper voltage limit), or you will need to regulate the voltage going to the bulbs. You will need a low drop out regulator (LM2940 should do the trick) otherwise the voltage drop across the regulator will take the voltage too low for the bulb when not charging.
If you don't have the space for a lead acid, then you will have to use lithium, NiCad or NiMh and decide whether you're making your own LED arrays and drivers, to run at the standard battery pack voltages (so you can use ready made chargers), or making higher voltage battery packs and charging systems to run the 12v bulbs, but you must use a properly designed charger for the specific battery type, to avoid destroying the batteries, or possibly causing a fire/explosion. Again, it may be worth buying a multi-LED or CREE Cycle or Caving head light, and using the parts to make your Lamp.
Whatever you do, I don't think USB power is very useful to you