I know of a 503 nearby.
If memory serves, it is new, in that it has never been started - just sitting on its wooden pallet for a while and
has oil injection, a B reduction box, electric start, dual ignition / dual carb setup. Owner may also have the carbs and possibly exhaust setup ( springs, hooks, muffler, connectors etc. )
PM me if you are interested and I should be able to verify ( or correct :nervous:/ modify / deny-part(s)-of !! :gig: ) what I have stated above.