That is without a doubt a mid-70's moped NOT 1950's but we have all established that. As far as plain gas in the engine....if you can get it running, mix it with the two stroke maybe it is not damaged forever.
I'm just going to repost the follwing text i used in the recent 76 PACER thread becasue this looks like it's probably a morini which would use the same ratio:
Basically what you want to do is mix roughly 4-6 ounces of two cycle oil into one gallon of gasoline. Use a gas can and shake mix it well before putting it in your tank. The reason i say 4-6 is the pacer manual says 6 and the morini engine manual says 4 so anywhere in between should be fine.
I am also a Pacer owner and this is what I use. Starts every time. You can find the two stroke oil in any auto store. If you are looking to change the engine oil at some point, SAE 30 (non-detergent = ND) is recommened for summer months, and SAE 20 (ND) for the winter which I've only been able to find at Walmart.