Could be vintage, mine used to have a Mac motor running alcohol, but some of the guys did run the Briggs. Smell the gas tank to see if it smells like varnish, could be your problem if they've sat around.
Could also be just backyard carts, not worth nearly as much.
If it's a 2 stroke Mac motor it needs mixed gas/oil like a boat motor.
Now that I think about it, mine had no serial no. tag on it either. The old style with the motor on the back we called an A-bone, if they had a motor beside you we called them sidewinders.
On ours running alcohol we could hit 90 mph in the straightaways.
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