48V 300W DC motor e-bike charging or running directly using solar panels. Is it possible?
Your best bet is method C:

Use your solar panels with an MPPT charging circuit and an output voltage the same as the existing AC charger.  Use that to charge the existing lithium battery.

That's the safest, surest, most efficient way to use solar panels to power your bike.

You'll probably have to let it charge for several hours in the sun.  Bigger panels means a faster charge, but more junk to haul around with you.



Method A could work, but the controller on your bike might not play nice with lead acid batteries.  Besides which, lead acid batteries are heavy.  You'll have to drag those heavy suckers around even when they are discharged - no fun at all when you have provide all the power through the pedals.

Method B is right out.  Solar panels can only supply as much power as there is in the sunlight falling on them.  That's about 1000 watts per square meter - at noon in the desert.  Everywhere else is less.  

Solar panels are maybe 25% efficient.  If you had one square meter of 30% efficient panels, then you could just about deliver full power to your motor from just sunlight - if you could keep the panel in full sunlight, and the wind didn't blow you over.