My documentation could be incomplete, but I find a 15HP Wizard in 1979, but if you can locate the model number, we can track it down. During that time period Eska was manufacturing the Wizards for Western Auto, and they were the same as those branded as true Eskas and Sears Gamefishers. Eska did make a 15HP water-cooled outboard, but I only see a 5HP and 7.5HP model available under the Wizard brand in 1979 ... but I could be wrong.
The challenge with the Eskas from this time period is that some of them had "Solid State" ignitions, and the replacement parts are nearly impossible to find if that is the case. If you have a traditional magneto, it is a bit easier. Eskas, in general, aren't the easiest to get parts for, but they can be had.
Some sources include Discount Marine, Ebay (look for Eskaman/Sanford's Goodies store), and Certified Parts Corporation. If we can figure out the equivalent Sears model number, Sears even stocks some of the basics.
Let us know your model number and we can help you determine what exactly you have and go from there!