Wanting to Upgrade Razor Dune Buggy - Help Needed, Please.

We got my son a used Razor Dune Buggy for Christmas.. Drove 1.5 hours to get it.. When I got there, the guy told me he just fried the controller 10 minutes prior to me getting there because he was rushing and hooked wires backwards.. He was honest and said he would give me a working controller he had and it was just an easy swap out..

Well I swapped it out and it didn't fix it.. So, I got a new 24V controller off Amazon and it fixed it.. However, he included a 36V lithium battery pack that he also admitted he didn't test.. Well after texting him back and forth after my son was riding it, he said the battery was shutting off due to low voltage cut off..

He ran this cart with a different 36V battery and said he never had any issues.. Annnnnywho, I'm stuck with a cart that intermittently works and I want to handle it and also upgrade it right..

I've seen people put huge battery packs (3x12v or even 4x12v) but those seem to require extensive modifications to place the batteries in spots where they will fit..

I saw a member on here upgrade his to 48V by swapping out the motor, controller, battery, and throttle.. This looks to be like a $350-400 upgrade (and I paid $250 for the cart already).. I'm willing to do it, but just hesitant because of the cost and also because I'm not sure what all will be involved to make things fit..

The price difference between a 36V controller, motor, battery, throttle vs a 48V doesn't seem to be much difference.. Can anyone recommend something to get me back up and running?

The controller I have now I believe definitely needs replaced because when I probed the motor wires from the controller, they only output 25v even though the battery itself was testing at like 41v..

Thanks in advance..