Hi again
perthmower, and

on becoming a Scott Bonnar Model 45 owner!

The handle assembly and parts list can be found...
HERE Your machine is a twin rail (Series 2) 17" cut, manufactured at the Thebarton factory in South Australia.
The Briggs and Stratton engine is an 8 cubic inch, which makes it a 3HP, with a Vacu-Jet carburettor manufactured on the 20th March 1976.
Allowing time for that engine to be freighted from the USA and fitted to your machine, we can hazard a guess at the manufacturing date as mid to late 1976, early 1977.

To allow us to assess the condition of the chains and sprockets, we need a close-up pic of the inside of the chain-case, particularly the sprockets where they are not covered by the chain.
Could you please also post a pic of the cylinder reel (close-up) so we can see how much life it has in it.

This machine will restore nicely, and if carefully maintained, will give you years of mowing pleasure.