These old Evinrudes run on and on and on. Simply follow the OEM set up.
Ive got one from the 60s that I use extensively ... and it shows virtually NO internal powerhead wear after ~50 years of hard use pushing a dink.
You can still get 'most' of the common parts for these old Evinrude & Johnson engines from the Bombardier/Evinrude dealer network:
www.evinrudeonlineparts.com
The way these engines were built, your grandchildren will probably inherit them from you ... if you just do the simple maintenance.
Secret - when long term storing: fog the combustion chamber as you run it dry, open the carb bowl or remove the high speed needle valve screw and with a syringe 'shoot in' isopropyl alcohol to remove the seasons accumulated gum/varnish, then drain and let air dry.
When removing the high speed (and or low speed) needle screws ..... count precisely how many turns (or fraction of turns) 'in' to full closed from your operating setting .... then remove the needle screw. When finished 'rinsing and draining', reinsert and reset based on the exact number of turns 'out' from full closed as above. Dont force the needle screw(s) to fully closed.
Also, every few years remove the power head from the 'bottom' and apply waterproof grease to the SPLINES on shaft that drives the 'bottom gears' and into the 'nose' of the crankshaft, every year if hard used. Will stop/prevent any corrosion/rust from occurring in the mating connection to the
crankshaft - the ultimate 'destroyer' of small Evinrudes.
If you live along or near the SE or Gulf coast, VA to Tx, use ONLY marine grade gasoline - eg. - from a Valvtect Marine Fuel Dealer. If you must buy the common (awful) 'ethanolated' gasoline used in automobiles, buy your fuel ONLY from a high turnover (busy) gas station, and then (must) use a stabilizer.