Old ElecTrek needs new battery

[Judy72]

I have an old ElecTrek electric assist bike and I need a new battery pak for it. It appears to have 14 .050000mAH NICD batteries. Can I buy a battery pak somewhere or substitute something better or should I get the battery replaced? You can probably tell that I don't know much about this!!
Any help would be appreciated!!
Thanks!
Judy72