Bought a Razor MX350 with dead batteries. Switched over to Lithium batteries. No need to change the controller or switch but did have to switch to a lithium battery charger.
This upgrade came in right at the $100 mark here in January 2024.
Battery: NERMAK 2 Pack 12V 10Ah https://amzn.to/3RZDt6H
Charger: https://amzn.to/3NLTs70
Disclosure: We have a V39 MX350