Hi Kim,
If it is losing air, it is probably because the seal has worn.
The additional weight of the rear wheel hub will not affect the suspension unit whatsoever, as the additional weight in the centre of the wheel is all acting directly on the ground, and is therefore not adding to the forces that are already compressing the suspension unit.
However, the weight of the battery pack will be placing additional load on the shock absorber, just like a heavier rider sat on the seat would.
If a seal kit is not available, or the unit is not designed to be overhauled, you will need to find a replacement unit with a similar amount of travel and the same fully extended dimension.
Alan