G�day Dave,
I too live on the Mornington peninsular, and would recommend contacting mal at all small machinery in Somerville (5977 9506), or Hastings mowers (5979 1382) as they are fairly cheap when it comes to second hand items.
If the starter motor is the same as 12hp ones I may be able to help you out if you get stuck (providing I have one to suit).
Hope you have some luck!
Regards Jay