Fuel "additives" are mostly scams, todays version of snake oil. Pump gas used to contain tetraethyl lead to help in the lubrication of the valve guides. Today we have better oil. You don't even need premium gas because the compression ratio is quite low on the old 750's but, I use it too because higher octane = more power and these old bikes don't have a lot HP compared to modern bikes.
If you live in a small town with a local airport, you can go there with a gas can and purchase some avgas 100 (100 octane) or possibly find some avgas 100LL (leaded 100 octane). Be aware that it is illegal to use these fuels in ground vehicles and they will cause your engine to run at a much higher temperature.
Note: The one fuel additive I personally know to work is Techron made by chevron. A bottle a month keeps the fuel injectors clean in modern engines.