There really isnt anything that you can beef up on them. Diffs for both versions are full metal, and, from personal experience, even the brushed diff can take 4S power, as long as you arent too crazy with it, but, 3S brushless it can take all day long. I had both the brushed and brushless...
It literally takes almost nothing to upgrade to 3S. Just the motor and ESC. There used to be a version that was 3S from Losi: https://www.losi.com/product/1-10-22s-kandn-2wd-sct-brushless-rtr-with-avc/LOS03013T2.html , but, they discontinued it. However, the brushed version has the exact same...