I just got a 75 Harley gas golf car and the oil/gas mix ratio instructions were inside the gas tank access door. It said to mix 1 oz. of Harley Davidson golf car oil with one gallon of gas. Use regular gas only. It also said, " As a temporary substitute, 2 1/2 oz. of a 40W non-detergent motor oil can be used".
I would contact a Harley dealer to see if any good two-stroke oil can be used and to find out what the proper ratio is. I.E. 50:1, 40:1, 32:1 etc. That's what I'm trying to do now.
Out of curiosity, what did you have to give for your 78? Is it a 3-wheeler?
Hope this helps,