What are the specs for the original charger? And what are the maximum charge rate (amps) specs for the bike?
We'll need those to help you find a matching charger for your needs.
Note that whatever charger you get you would probably have to change the connector on it to match that already on the bike, if the present charger isnt' built-in. If it's built in you'd be replacing that with the new charger, and so you would be wiring it directly into the bike in place of the original.