not sure how it is on the older bikes but on mine its easy stuff, air filter is super easy to get at.. just lift the tank n remove the airbox lid, oil changing is easy enough, take the side fairing off and drain it all out.. easier if you have warm oil (preferably not so hot your hands melt when you touch the oil..) but i generally leave it for a few hours to drip out, the filter has its own little cover which unbolts easily enough and the new filter just pops in, putting the new oil in can be a tedious (or very messy if you hurry) job and dont forget to make sure you dont overfill.
spark plugs were a serious hassle for me at first but i have since managed to get them out without doing anything but lifting the tank, if i remember correctly it tells you how to do it in the aprilia workshop manual (pretty sure i saw a link to it in the file vault)
chain and sprocket change, what fun, i did that recently, you gotta take the rear wheel off n mess around with the sprocket carrier etc, i guess its not the worst job but it can be time consuming and dont forget to wash it properly or you will stickier than a chimpanzee who took a bath in the local tar pit
you wont regret changing the gearing though :thumbup