yes where the starter cup fitsNo a 503 is not going to fit. Are you looking for just the aluminum part that holds the magnet ring? It also has the steel taper hub in it? Is your bolt holes stripped where the starter cup bolts on? They make step stud in that I've used to tap the flywheel to the next size and it will still fit the starter cup. Then I lock tight the stud in the flywheel.