Hi Peter, welcome to Outdoorking.

Parts and part numbers for Kirby-Lauson engines can be a bit challenging. You probably know that Kirby imported most of the parts from Lauson in the US then did final assembly, plus sourced some bits locally, to avoid import duty on those engines. However Kirby mixed-and-matched features from the Lauson model range so his products tended to be hybrids of a couple of Lauson models.

Here is the Lauson Parts List that applied to the engines Kirby was mixing up:
http://lausonsmallengines.com/literature/specifications/tecumseh/H.PDF

As you'll notice, when it comes to crankshafts the parts list tells you to refer to Form 690157 dated February 1958. I don't know how you would get hold of that form, but it probably includes lots of different part numbers for crankshafts that only differ in details that don't matter to you.

Be sure you select a crankshaft with a stroke of 1 3/4", which means it will be for an H20A, H25A, or HB25A. The H30 engines had a longer stroke. Note that Kirby-assembled engines were identified as HK instead of just H.

Incidentally, you might be interested in this thread:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=37736#Post37736