Yep,

I have no doubt of the engine's origin - a Iron Horse C12B.
The three lines on the starter housing are a dead give-away.

I need to say something about the tapered cowl though.

It was typical for small outboard manufacturers to change the cowling.
One reason was practical - a larger fuel tank; the other cosmetic.
The Kirby engine was mounted transverve on the mower frame.
Here the cowling tapers - streamlines - towards the back.

So, if we can establish it is an Australian outboard, there is a good
chance the engine is a Outboard Marine Iron Horse made here under licence.

If not of Australian origin, then it will be a simple Outboard Marine engine.

[Linked Image]

So' let's wait and see.
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JACK.