Battery for electric start

The new batt has to attach to the bike like the old. The plug needs to match. The controller needs to be compatible. Don't just buy any battery, most have no return policy. Be sure of what you're buying, or you'll be throwing away good money. Is this a kit added onto the bike, or an electric bike built OEM from the factory? Show us some pics of badges on the bike, maybe we can help you locate the proper parts.
OP did say "electric start" not electric bike. He needs a cranking battery.