Unfortunately solitaire's are not very mod friendly, as the reflector and lens require brute force to extract. This resulted (in my case at least) in dammge that meant, whilst i got them back in place, any further mods are impossible.
However, if you can drill the reflector out to 5mm, you could fit an LED in there.
This is the thread that inspired me:
http://candlepowerforums.com/vb/showthread.php?t=102083
I would recommend doing something like this using the a23 lithium battery.
Failing that,
LED replacement do a tektite LED drop-in, not sure of the quality though.
Happy modding!
Scythe