Most of the sets I used as is. I did have to move the solar cell.
On some light brands I threw a lot of plastic in my recycle bag. I trimmed the plastic part that held the batteries and circuit board so it would fit in small buildings with room to spare.
I cut the red and black wire going to the solar cell and added about 3 feet of small (26 gauge) brown wire I scrounged from something. I made sure I had the polarity correct, red to red and black to black. I put some contact cement on the wires and solar cell to keep them from pulling off.
On later conversions, like this one, I added little connectors (Radio Shack 276-1388) for the wires. This made it easy to hide the wires.