You have a 1977. The mystery fluid is Type F Auto Transmission Fluid. You just fill it to the top of the screw hole or thereabouts. If you want, there is a drain plug on the bottom of the engine and you can drain out the old fluid and replace with fresh - it only takes a few ounces, so a qt of Type F will last a while.
Some handy items;
You can go here to get copies of service and parts manuals for your Puch. Don't be concerned that is says '80-'81 - they'll work, and they're free!
There is a ton of info on the Wiki here on Moped Army. The E-50 Rebuilding Guide is a great picture view of the engine (E-50 is what you have) so that you'll see what's inside - nothing to be afraid of!
Welcome, and good luck!