Okay, here's the plan:
I've uploaded a diagram of your venting system. Check out my personal ALBUM (If you're using internet explorer, hold down the SHIFT key and click ALBUM at the end of my thread. If you're using Firefox browser, hold down the CONTROL key while clicking on the ALBUM button. This will open the album in a new window) Use this diagram to help you figure things out.
Assuming you've determined that the fork is indeed magnetic (made of cobalt, nickel or iron) by testing it with an identical fork you may already have in your kitchen and driven up the steepest hill in your area that you know of in order to slide the fork as close to the vent opening as possible, you'll have a chance at getting the fork out all by yourself. Before you give yourself a pat on the back,.....
GOOD NEWS:
The venting duct turns 90 degrees upwards meaning that your fork must be located between that 90 degree turn and opening of the duct where the fork originally fell in. This means you've located the fork. It's not lost.
MORE GOOD NEWS:
The venting duct from the diagram appears to come in 2 pieces meaning you will not have to cut it to gain access. You may be able to simply pull the two sections apart. But first....
BAD NEWS:
You'll have to remove the passenger seat to gain access. Sorry there's no other way unless the magnet technique really works, in which case "congratulations." Otherwise, stop celebrating and continue with removing the seat. Though i've never done this, I assume you can remove the seat with a few turns of a wrench and some strong arms and peel off the carpet to find the duct underneath. I know this may sound scary, but it may be worth it.
Good luck.