No need to ”reinvent the wheel“ just rebuild the kart per the original specifications:
Murray Hard Drive parts manual
*ETA: I see that the rim and tire sizes are not listed in the parts manual, only a stock number is given;
#39 Tire Assembly--Rear 1201274
#33 Tire Assembly--Front 1201273
My suggestion is to insert "Murray Tire Assembly Rear 1201274" into your search engine of choice and buckle up. There are some hits out there but you may have to deep dive on the Murray Hard Drive kart, even looking at tire photos to find out the tire sizes. Hint: they are industrial grade and are often used on riding lawnmowers and similar applications.
Maybe someone here knows already?
@JimD
You wouldn't want to disappoint these nice ladies, would you?

Upload some pics so we can see what you are working with and if it is in original condition and not some Franken-kart stitched together by Bubba with random body parts.

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