You can but you won't see any kinds of huge gains from modifying this engine. Don't get me wrong, I LOVE the 22RE's. But they are expensive to really make them pump out some respectable numbers.
But if you are determined to do it, here's an awesome website.
http://www.lcengineering.com/
They carry all things 22RE and then some.
But there's things you can buy at the local Parts store like better Cap and Rotor, better Spark Plugs, and Wires.
A tune up never hurts!
Good luck with that!