Well, Ive had a bit more of a look and taken pics.
Pretty sure the Kirby is a 3/4 shaft and the Briggs a 5/8...

You cant really see from the pics of the Kirby's.

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So am really not sure if its worth the effort or the new LHS piece to put an old B&S on it...

Ive seen a few new engines on evil bay, brand new, thoughts ?
http://www.ebay.com.au/itm/Briggs-S...pt=AU_Lawnmowers&hash=item1e706759a1