Rotax 503

3682426 was produced 03-Mar-1987

that's all the info the factory database has on it, nothing else is listed - not even the engine type code. Tuesday I can check the ancient North American Distributor paper archives, we might find a little more info on it there.

You can find all of the available manuals for it here:  https://www.rotax-owner.com/en/support-topmenu/engine-manuals#2-stroke-engines

A look through the North American Distributor's ancient paper records confirms that is is indeed a Model 503, (sold to a ROTAX dealer in the USA) but those records give no further technical data on the engine...