Ggripfreak89 is partially correct. I own an Axis and I have a big list of hop-ups. First off, the AXIS is a FANTASTIC starter buggy into Nitro RC for one reason: it will teach you the good and bad of nitro RC. Lets put it this way; if my first buggy was a Kyosho Inferno or some other $800+ buggy, I'd be flat broke allready repairing the thing and would never reach the buggies potential with my experience for a LONG time. The Axis was never produced to be a competition racer, more like a weekend basher (even though that never works out either). I'm about 75% happy with my Axis. I've hopped-up mine so much that other people at the LHS ask me what kind of buggy it is.
The hubs are 19mm. Big problem for options on wheels. Go to Tower online and get the Ofna 17mm hubs and nuts, they swap out with the Axis 19mm easily. Now you have a huge range of wheel possibilities. You'll notice off the bat though 2 things: 1. the Torq .21 engine does suck a bit and 2. the baseplate for the car is flat vs. the one found on an Ofna which cradles the guts of the buggy, so it tends to get pretty dirty.
Now back to your inital question: Hop-ups. Heres what I've upgraded--->
front and back adjustable Ofna shocks
Ofna White springs all around
Ofna 17mm hub swap
Kyosho gunmetal wheels
Proline Badlands tires (bashers dream tires)
Futaba metal gear servos
3 shoe allum, racing clutch
tuned polished pipe
longer fuel lines
purple cooling head (to look purdy)
custom painted Ofna Violator lexan body (the Axis body is hideous) by yours truely
Ofna Shaftstarter system with Ofna 5011 backplate (fits perfectly)
custom CNC front shock tower (no real diffence than stock)
I think thats it...
What irritates me the most is that people dog on the Axis so much, and from their reputation, they deserve it. I love it when I whoop up on an Ofna buggy at the track and the guy is like *** is that thing??? But with many things in life, if its upgradeable, keep it, untill your skilled enough to go bigtime and spend $400+ for a better buggy. Things will break and the engine will piss you off, but you'll learn alot from it, as you would with any other buggy, but for its cost and its potential, an Axis is a great buggy for starters. I myself will be buying a new buggy soon and it will be something raceable, but from my experience from the Axis, I know what to look for. Feel free to PM me whenever you have a question on yours that I may be able to help you with.
P.S. sorry for the lengthy reply...