well....
Solar will help a little, but don't expect getting a full charge. That would take a couple of weeks.
assume you have a 36V system.
100watts/36V= 2.7 amps at full sun. Not a real lot of amps.
Your 120V charger is ~15 amps and it take how many hours to charge your battery?
Just set you expectations realistically. Get the maximum power solar panel that will fit on you cart.
Yes, I am going to add solar to my cart, to give me a longer run.
If you add a solar cell(s), get the voltage at MPP the same as your battery system at full voltage.
Make sure you use a solar controller or at least a series diode in series solar panel so it does not back feed to panel at night and drain your batteries.
Good luck