Furyan, Given you live in Sydney, happy to help you do this if you're set to change it to shark fin.
The change will require to purchase a shark fin cover for the E91 and have it painted to body colour. To install the shark fin cover you need to first remove the base for the rod antennae, you can actually remove the cover which will make the original base fit inside the shark fin cover. To fit the shark fin cover you can either use sealastic or screw the cover.