That model number can't be right. If it's actually CN instead of ON then its a 1982, which I believe is the first year of that motor and is often not listed as a valid model number for whatever reason.
you should start a new thread this one is very old. but too your question if they offered an electric start for 20 hp in the same year than yes you can. at the very least you need a starter a soleniod and if your flywheel does not have a ring gear with teeth on it you will have to swap it out. also you will need a starter bracket. and i almost forgot for safety you need a neutral safety switch. so that it wont start in gear.
