I had one in my garage for 12 years and it had been unused for who knows how long before that.
In less than an hour of oiling and putting on a new belt it was a running machine. There is likely nothing wrong at all with that 66 other than it's dried out inside. Use real sewing machine oil in the machine, I like the Singer stuff when I can find it. Don't forget the joints on the treadle. They need oiling too.