I converted my 2003 36 volt series to 48 volt Navitas and I'm not sere of exactly what you have. On mine, I needed a F/R switch (SPDT) which I mounted over the hole where the original lever was, right by my right knee. I also needed a run/tow (SPST) switch because mine never used one. I extended the wires from the harness for this and mounted it on the dash. I used a heavy duty solenoid which I sourced from ScottyB. I needed a new battery charger and after a little problem I had I ended up with a Lester Summit II which works great. As for batteries, you get what you pay for. I had 6-6 volt Trojan T-105"s that gave me good service and now have 6-8 volt Crown CR165's (recommended by ScottyB at one time) and am happy with them. Right now with the supply chain issues it boils down to what is available. I would have went with Crown CR190's if I could have found availability. I previously had charging issues with a DPI charger because it didn't have a charging profile for Crown CR165's. And don't forget #2 gauge cables. I also posted a picture of how to arrange your batteries so you only need 1 additional 9" cable if you're re-using the set from your 36 volt setup.
Here is a link to a post that I posted after my conversion.
https://buggiesgonewild.com/showthread.php?t=178084
https://buggiesgonewild.com/showthread.php?t=178084