Welcome!
To start look on the blower housing for a sticker or model number stamped into the metal.
It should say something like lh358sa-XXXXXX or hmsk100-XXXXXX
Unfortunately the 6hp cranks stroke is too small, and just will not fit.
Also Tecumseh went out of business years ago, so parts support ended.
If you do find a NOS crank , its gonna go for $150+
If I remember correctly the # 36245 crank should be what your after. Here is an example used ,
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Really it's only worth using the motor if yours in in great shape, and you find a donor with a good crank.
Or get a quote from a machine shop to turn yours down to 3/4
Another option is to run an adapter sleeve from 7/8 to 1in, but it will only
work if the clutch has a female keyway, that limits your options greatly.
Clutch EBAY
adapter EBAY
But, in the end, I think your should find a different powerplant for your kart, something with parts support.
Recently the predator 8hp was on sale for $280.