thanks Deejay and Grumpy

Im 99% sure the Kirby is 3/4. I could fit a 3/4 spanner between the inner clutch and engine, and was the right size for the shaft. Its also significantly bigger than the Briggs 5/8...
So couple more questions
-if I chose to go the inner clutch change, how do I remove current and refit (I can take pics of the captive cotter, is this the screw that holds the inner clutch to the engine shaft (is it also called the Engine Shaft Clamp pin???)
-secondly, Im leaning towards not relying on my old Briggs, as I not confident the old engine is reliable etc.. What/where/how much would other options of replacement be (vs the Chinese Briggs) ?

thanks again.