What kind of a shaft? Or are you looking for an axle. The two most important factors are length and diameter. Huge selection available from most any of the major go kart supply shops.
Great! Thanks for the link to the manual. I’m actually looking for it. I should add that after saving, my PDFs started displaying incorrectly. So I used the PDF Guru converter https://www.youtube.com/@PDFGuru I came across recently. It supports a wide range of formats and is honestly quite easy to use. For now, this has solved the problem, and I still need to figure out what to do next.