I've now cut down three of these on maxi scooters, including the MP3. All you need is a jigsaw (you might be able to use a reciprocating saw or even a Dremel) and some sandpaper. My method:

First figure out the height and shape. My practice is to take a piece of black electrical tape and put it on the windscreen where you reckon you want the new height. Ride for a day, then adjust up or down (and change the shape of the curve) as necessary. Remember you can't make it taller once it's cut, so err on the side of 'too tall' and you can always take it down some more.

As an aside, you can make a very smooth curve with electrical tape. Start by just tacking it straight across the screen. Mark the centre point, then pull up one side of the tape so its still attached at the middle. Then just work like a pinstriper; move the unattached end of the tape up and down while your other hand smoothes the tape along the screen from the centre out. Very smooth curves can be done this way with just a little practice. Then do the same with the other side from the centrepoint.

Once you've fixed the optimal height and curve with the electrical tape, use masking tape to cover the top 3 or 4 inches below the masking tape (to protect against scratches from the saw). Leave the screen on the bike while you cut it - it will be held more securely than if you take it off.

Take the jigsaw with a wood/plastic blade and start cutting from one edge all the way over to other. Pause every couple of inches and let the blade cool down or you will start to melt the plastic which makes the sanding more difficult later. Keep a good grip on the top of the screen while you cut; if the blade 'bucks' you can split the screen and that is bad.

When you've cut it off, leave the masking in place and take it for a ride. If you can see ok over the screen in everyday riding that's good. My preference is to have the screen high enough to direct most wind over my head, and allow me to duck down behind it, but low enough so that in a normal day most of my field of vision is above the screen.

Anyway, if you think you've got the height right, start sanding, always running the paper along the edge rather than across it. Round the edge slightly like the stock screen. I begin with 100 grit to get rid of any major flaws, then work down to a finer grit, ending with about 320. The finished edge looks just like stock. If you want to go further you can make the edge like glass by using 400- and 600- grit and then finishing by rubbing with a silica-based cut-polish like Silvo or Brasso.

A word of caution - you don't have to mask the back side of the screen to cut it, but if you don't, be very careful with the sandpaper.