Kostas, what you can do to keep it relatively simple, but still get a nice increase in power would be:
* get the 12.5:1 compression 249cc JE piston (no need to bore up, just plug and play) they also have 13:1 compression piston, but you need to run high octane fuel.
* get a juice box / power commander, and adjust the timing and fuel delivery of your injectors (IF model)
* a good exhaust. if i had to choose any exhaust now, i would get the AreaP racing exhaust:
i would love to get this exhaust, but there are 2 problems for me... no pasanger foot peg, so GF would not be happy, and i already have an AP exhaust and my GF would be doubly not be happy if i spend more money for something that i sort of already have, LOL...
* wide band AF Sensor, so you can tune the bike without having to go to a Dyno.
* Open air filter such as K&N (maybe if removing snorkel is not enough)
hmm... that's about it for starters
should keep you busy for a while~