Where do I start?

Typically the cranks are either 3/4” or 1”. Measure the clutch and you’ll know which to get.

If it’s a 1” clutch you can also get spacers/adapters to space a 3/4” clutch to fit.

That said getting a new clutch is a good way to go in case the crank on the existing engine has some unusual length or taper. Looks like there’s already a spacer on there.

In that Porsche fun kart thread, the guy used a 5hp Briggs and Stratton with an electric starter. Harbor Freight also sells a wide variety of clones if I’m not mistaken.