Hi
My battery pack is also knackered.
I have bought some yuasa lighting packs which are nicad D cells from Ebay. They're not the original 4500 mah ones but 4000 mha cells. I'll give them a go as I bought them in sticks of 5 cell pre soldered packs, with tags and so easy to solder up into the required 20 cell pack for the Zike.
They cost £29 with delivery, that's my new battery cost and so worth a shot.