Once you get the cart home and access the situation ;
First - how old are the batteries in the cart ? Batts r the heart of the cart
If they r over 5 years old replace them they r probably near there end of life span , if the batts r less then that and r in good shape
Put a newer controller in 400 or 500 amp keep with the 36 volt system until
Your original batts die then upgrade to 48 volt and new batts , I would upgrade the solenoid to one that can handle 36 -48 volt as well
Good luck
Cheers !