The old charger was a 25 volt 50 amp unit (!! 50 amps !!), but I am going to use an external 24 volt 40 amp unit for the LFP battery. There will be a charging port under the seat, not decided what plug / socket to use yet. Maybe a waterproof design?

There is a possibility of modifying the old charger to put out the right voltage, and having a cutoff mechanism to stop charging. I'm assuming that it's a CV / CC unit.