If you can't get any replacement decals (I'd try that first) Then you may be able to print out the Ducati logo in the correct size using your inkjet printer.I did this myself on a Yamaha YZR500 kit when I ruined the tank decals.You can get the image from Brands of the World's site and print it out on clear decal paper.You need to seal the decal once printed and dry,then cut out and apply in the normal manner.
http://www.brandsoftheworld.com/sear...ch&text=Ducati
The best option would be a replacement sheet of decals though,if you can get one.But I haven't seen any in 1/9th scale.