A good thought Paul. AA or AAA battery boxes may be easier to find on places like ebay and some come with a switch built in, which can be useful. Being black, to my eye they blend in with the underfloor boxes, brake rigging and other stuff, and are easy to get to when changing batteries.
It might be worth trying a couple of batteries on the wires, in case the previous owner has lit them with LEDs when they fitted or changed the lights before taking the roof off. You may be lucky and find that they have been changed already, as finding space for 12 1.5volt batteries could be a problem to do neatly. You may also find that you don't need the full voltage on 18v lamps as I have seen some track powered carriages where the whole roof and bodywork lights up with the lights on inside when the full voltage is applied to the track. LGB say there are 4 lamps in these, so that could be too bright to look good. I found that two small LEDs is quite bright enough. I have used some translucent plastic over the LED, and reflective tape (as used to tape over joins in 'Kingspan' type insulation), I also cut a short slot through above the LED, into the underside of the clearstory part (on coaches fitted with these) so that there is a little light shows there too, as the real ones do. Well it looks nice to my eyes anyway. I will try to remember to take a photo when I go outside later, I hope that helps. David