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To covert an Oldham make a flat plate of aluminium or SWG 22 copper to fit into the lamp after dumping bulb holders. Mount drivers on the rear and LEDs on the front.
Best for low voltage are 7135 based drivers, but beware of silly flashes and strobes. You can remove the anti reverse polarity diode which wastes 0.33 volts (assuming Schottky) off whatever battery is supplying. The drivers are for torches and the diode is to safeguard against silly buggers putting batteries in the wrong way round. Standard diodes waste 0.7V.
Each 7135 gives 350mA, so they paralleled. For a multi mode you will get about 50mA on top off the microcontroller.
Another good supplier is kaidomain, they do a lot of the stuff without the flashes, or you can solder a little bridge to get rid of them.http://www.kaidomain.com/
If is a 7135 based multimode, you can take one 7135 out of the microcontroller to set a realistic bottom brightness level.
Remember that if you a first timer when you start modding stuff you will start trashing stuff, so best buy in a few of everything.