If only few cells r bad, then repair can work, but mind it, u need precision equipment to match cells , cells need to be in 0.1volt variation with each other.mostly used batteries are nothing but bad batteries with cell replaced nd labelled as fresh imported batteries.
If the whole pack has degraded, repair wont work nd its ur hard luck to find a used battery which can work.
New modules fresh from factory is best option but very few people willing to pay price, at current dollar rate, 2lakh for aqua. I had some modules custom designed for this purpose nd fitted in few cars in my circle, nd they r working gud. Have a sneak peak. (My line of work involves with LiFePO4 batteries for purposes i cant write here)
8point spot welds to handle high currents nd keep module internal resistance low
