The problem could be
mixing old SLA batteries with new ones. You can read the technical details here: https://electronics.stackexchange.com/q ... -batteries
But the short of it is that the newer batteries put out a stronger current than the old ones, frying the old ones. You will have to disconnect all of the batteries, drain them completely, (hook each battery up directly to the motor and run them down), then recharge them with a 3a or less charger. Check the voltage at timed intervals, (I usually go every 30 min), and see if they charge at about the same rate. If you need to replace the batteries, eBay is usually your best choice:https://www.ebay.com/itm/12V-10AH-Wheel ... SwNuxXaYKk
Although Walmart seems competitive:https://www.walmart.com/ip/12V-10AH-Bat ... 3=&veh=sem
If your batteries are OK, that leaves your throttle or controller. Reading through your post, you don't mention if you upgraded the controller. If you ran 36v through the stock controller, then it's probably dead. If it was me, I would rip out the controller and replace it with one of these:https://www.ebay.com/itm/391750654895?s ... 1436.l2649
(Although to be honest, I generally rip out the controllers right away anyway!
). It will take about two weeks to arrive, but IMO it's worth the wait. It comes with a replacement throttle that plugs directly into the controller then you only have two wires for the motor and two wires for the battery to hook up. I know that the ad says 48v - 60v, but I have used one on my daughters Pocket Mod with excellent results:viewtopic.php?f=13&t=3286
Your other choice would be this:https://www.ebay.com/itm/36V350W-Motor- ... H-&vxp=mtr
although, if you can wait 2 weeks, you can get the same thing for 1/2 of the price here:https://www.ebay.com/itm/Brush-Motor-Co ... fA&vxp=mtr
Modding scooters can get to be pricey, so planing out your projects and ordering from China is the less
expansive way to go!