G'day Alex,
Welcome.
This is my take on it. When researching new engines for my little 14", unless I wanted a Chonda or similar, the only decent engine in 5/8th was a Honda GX100. I wanted to keep my original cast iron clutch, so that didn't leave a whole lot of choice!
The Briggs and larger GX motors were too big to fit nicely between my original straight bars without extension plates as well.
You can get Briggs in 5/8th, but not the GX series of Honda engines anymore. They're all 3/4 (19mm).
If you need a new or reconditioned clutch, go 3/4. If your flash new Briggs ever lets go you're "future proofed" and won't need to swap out a clutch again. That's my recommendation.
Since you're in Perth, check out Trident Lawnmowers in Swan View. They make new alloy and cast iron clutches, in both sizes.
