You can purchase replacement masts from Supercheap or Autobarn. Replacement is reasonably straight forward but there are some tricks.
You need to remove the inner mudguard lining, remove and dismantle the aerial, leaving the power connected. An assistant to operate the switch will be helpful.
Remove any parts of the broken mast. There should be a nut securing the mast into the top of the aerial.
Insert the drive cable into the aerial hole, engaging the teeth with the drive gear on the motor.
Reconnect the leads and have the assistant operate the down switch. Once the mast has fully retracted, the aerial can be re-asembled, mounted and the guard liner reinstalled.
Make sure all connections are tight
Test the aerial in both directions. It might actually eject fully. If this happens, you will probably need to go to an auto electricial to have the gearbox in the aerial correctly positioned so that it cuts out at the right time. There is a small timer segment on the drive gear that needs to be in the right position for the aerial to work correctly.