I would say that was very good detective work there. The 853A specification did go to a Qualcast 16" Sidewheel Powermower.
Here's the confirmation here:
I did write a History Record for these mowers, filed under Australian Power Reel Mowers (because they were made here) It would appear that the engine was fully imported (lacking the X number).
Spent today returning the 131X engine back to running order
Villiers Mk2 Midget engine
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The fuel tank is from a small JAP engine and the air cleaner is incorrect for this model. It should have the chrome trumpet type filter which I have but its soaking in the cleaner at the moment.