One option you have is to tri-cell with 3 1/4AAA batteries. The largest mAh rating I've found is 70 which is really lousy but the batteries are smaller than what comes with the ZipZap.
I saw at
http://www.bitmod.tk/, they have 3.6volt lithium polymer mod although the site is in German.
The following is the German translated to english:
Bitchar g with 3.6 V lithium polymer Akkupack of Microsash.
The Akku was mechanically deformed after arrangements with the
manufacturer. Fundamental the reproduction is recommended only to the
experienced Bitmodder.
For lithium polymer Akkuzellen is necessary a special load technology.
During inappropriate handling danger of explosion exists.
The ZipZap Mod:
J