For the purposes of getting it running, just install a 12V ignition coil and disconnect the BAT wire at the voltage regulator. You can crank the engine with 12 volts without damaging the starter and nothing much else will be affected by a short run at the higher voltage. Be sure to lubricate everything thoroughly, and turn the engine several revolutions by hand before running the starter. Repair parts for a 503 are as scarce as a chicken with lips, so don't take a chance on trying to force the engine to turn over with the starter if it's even slightly sticky. Engines that sit for long periods of time tend to get the piston rings rusted to the cylinder walls, and/or valves will stick in their guides. Good luck!
Jerry
