Can the lock for gas tank be taken apart and the tumblers rearranged to work with an existing key? Or would I need a new lock set?
Probably can. Remove it and take it to a locksmith shop and see what they charge. Or just get one of these:
http://www.motowheels.com/italian/myproducts.cfm?parentcategoryid=38%7CAluminum%20Billet&productID=392&showDetail=1&categoryID=45 (http://www.motowheels.com/italian/myproducts.cfm?parentcategoryid=38%7CAluminum%20Billet&productID=392&showDetail=1&categoryID=45)|Ducati%20Monster%20Billet&vendoridtodisplay=0&filterFor=&collection=168%7CEuropean%20Motorcycle%20Parts
The lock comes apart easily, but you probably won't have all the right tumblers. You can use some of what you have so the lock won't work with a screw driver, or you could just remove the tumblers all together so it will work with anything that will turn it.