An inexpensive battery replacement for the LC series is a set of two AA or AAA 1.5 Volt alkaline batteries, connected to produce around 3 Volt output voltage. The batteries could be mounted in a little battery holder, as available from Radio Shack. Wire the holder terminals to the battery connector on the mainboard (mind the polarity!) and fixate the holder in a place for convenient access, like the space around the speaker (use some adhesive mounting tape). Make sure not to block the fan with the additional wiring. This works very well in any of my LC475s.

If you make a connecting clip in the size of the original lithium cell, you can connect the new battery without any modification to the mainboard, leaving the mac in original condition.

P.S.: prefer to use "leak proof" batteries.