All of mine from that era run fine on unleaded, I just add some Seafoam in it, or equivalent. Rural King's supposedly "same as" is a couple bucks less per can. I'm now trying Stabil's new product that stabilizes fuel, and also cleans the fuel system.
The biggest issue here, is with Ethanol in gas. It will pull moisture out of the air, and into the tank. Using these products seem to absorb it, and take it on through. No real issues with fuel in the last 5-6 years I've been using those products, with specially in my small 1 cylinder engine on garden tractors from the 50's & 60's.
My IH 656 seems to run better on mid-grade/93 octane. It may have a little higher compression than the smaller tractors. Regular tends to foul the plugs, when on a harder pull on the rotary cutter, or, running the round baler. A local distributor told us that there is less Ethanol in mid-grade gas.
You may find a local airport to sell you leaded 100 octane Av gas, but right now, it's $4.75 per gallon here. Non-Ethanol Marina gas can be bought also, but here, normally runs between $1.50 to $2.00 per gallon more. I'll splurge on that to run in the chain saws, and other 2 cycle engine equipment I have.
