The Standard engine in the TEA20 tractors will run without damage on unleaded fuel.
I use Lead Replacement Fluid in all my old engines. THIS
is one example.
One highly illegal solution is to put a few round lead fishing sinkers in your gas tank. Rolling around they release micro amounts of lead into the fuel with the added advantage of also removing rust and lining the tank with a micro coating of lead. But as I said, it is highly illegal in most countries........ :D
Bob in Oz