Alright here's what you need to do. Get a wooden dowel that will fit in your spark plug hole. Anything that will fit will work really but it needs to be straight and not metal. Pull a spark plug (might as well pull them all and check them since it's new to you, plus it'll be easier to turn over). Insert dowel and have a friend turn it over by hand from the crank bolt. Mark the dowel at the highest and lowest point of the stroke. The distance between these 2 marks is your stroke. 3.5"=352/360......about 3 3/4" (3.784)=390.....if you happen to get about 4"(3.984)=410/428 <--doubtful but it would be a great find. You never know what has been swapped in.