Hi conquest. There is no way we can tell you what to replace in an engine without knowing what is currently wrong with it. We would also need some pictures of the mower and engine. As a starting point, here is a thread on overhauling an old Kirby-Lauson like yours:
https://www.outdoorking-forum.com.au/forum/u...s=Kirby-Lauson&Search=true#Post38144