Pete,
Ive done this kinda work before ... many years ago.
Mask up the line you want to cut along, then mark it up with a texta. Find the finest, finest cutting tool you can (maybe in your case a fine cutting blade on a grinder and/or dremel). Peal off the tape when you are done as soon as you can, because water and masking tape can leave the tape glue behind (and some of the paper). You will more than likely require a small touch up around the cutting edge.
If you need a good bloke for the touch up ... try Daniel Stone Paint and Panel at Hackham. He's an old riding buddy of mine and hes done work to my Stageas. He's done body work for custom Harleys as well as jap crap bikes Tell him ruby sent you and hell look after you.