Urpalcharlie, and or any newer rc enthusiast with similar experiences, this is just IMO or my experience, unfortunately I haven't seen really any up grades for the axles ie: knuckles metal gears or better motors. I have had really good luck and only broken links both suspension and steering and knuckles as of yet, I plan on using this thing till the electrics give out and then find some metal or 1/8 scale plastic MOA- motor on axles, like from bully or berg or aftermarket and build this up.
I bought mine about this time frame last year and started tinkering with it before Christmas, didn't want my then 5 year old son to smoke me right out of the gate, he's a fast learner and super competitive, any ways I modded mine almost right out of the box and have been running 3s and the upgraded servos the whole time, nearly a year now. Its been a great little rig, weight and bigger tires make for great traction but your drive train will break, you can get away with bigger tires but it strains everything, adding weight intensifies that even more so id choose one or the other toy with it and you find what works for you.
Also if you watch you tube vids watch some of the comp crawlers you got to know how to play it safe, force it and you'll probably break it, ie: binding of steering or in a deep rut and hammering the throttle, not to mention if it takes a nasty fall it will defiantly break. Its awesome for what it is, especially at its price point.
Big name brands will break too, so if you really enjoy it, id recommend starting a build, price things out and ask questions the process of building is imo even better then buying a complete rig. In the mean time replacement parts, shop around, and if you enjoy it alot and hate the down time, get multiple rigs their 120 bucks, almost cheaper than buying spare parts, if I hear or see a possible upgrade ill post it up and so will most on hear and youtube best of luck.
P.S. check out these on ebay
axle assm
https://www.ebay.com/itm/Redcat-Dan...951955?hash=item2d169d8b93:g:gxIAAOSw53xe9Vmh
whole chassis
https://www.ebay.com/itm/REDCAT-RAC...149298?hash=item421f8c9872:g:YykAAOSwk5pfKuso
gears buy them self
https://www.ebay.com/itm/Gear-Set-R...457425&hash=item3fbd6d5e3c:g:mEQAAOSwxklciqp6
gears buy them self (redcat)+ shipping
https://www.redcatracing.com/products/r5612?variant=30993827201114