You need actual charge control logic for a modern rechargeable battery so that it doesn't overheat or malfunction, but you can get multi-amp Li-ion/LiPo chargers online for ~$25 if you're able to find and attach the right connectors.
This also seems like a situation where the maker community could easily open source replicate whatever the custom connector to the bike requires.
You can't just hook up any lipo charger. They need to be able to charge and balance the cells individually, which could be simple or extremely difficult depending on the arrangement and connector.
This also seems like a situation where the maker community could easily open source replicate whatever the custom connector to the bike requires.