Is this your first bike?
If you are new to dirt bikes I would try just riding it and see how it runs. There is a good chance that the carburetor needs cleaning but I would at least give it a try before I started tearing it apart. If it runs OK then ride it. If it bogs, won't idle or runs poorly then clean the carburetor.
To correctly jet the carburetor requires a bit of work. There is no "right size" jet, but there will be jets and needle positions that are better for how your bike is equipped, the altitude you are riding at and your riding style. Again, if you are new to dirt bikes and it runs reasonably well as it is then I would leave it alone until your skills improved.
Once your riding ability improves and you start pushing the bike for performance then it would be time to rebuild the top end. The engines on these high performance bikes require frequent top end rebuilds (piston & ring) and if you don't do it you will eventually have a major failure that will cost a lot more $$ to fix. 20 to 30 hours on a top end it pushing it. If you don't know the history of the engine then you should plan on a rebuild before you start asking a lot from the motor.
As for modifications: Get your skill up and figure out what your riding style and type of riding is. A modification that can help in one situation will hurt in another. A great example is the pipe. You can change the pipe for one that provides more power at high RPM or you could put on a pipe that provides a bit more torque at low RPM. Which is better for you could depend on if you ride track or trail.
As mentioned before the overall best thing you can do is get the carburetor jetted properly. Note that changing the pipe will probably require a change in the carburetor jetting as well.
Rod