An option, if you want to keep it "original" would be to take the cowling from the 1971 and plop it on a 1980 or newer crossflow V4. Paint the lower to match, and you’ve got yourself an old "look" motor with newer components. 1980 is kind of a cutoff year, at least for me, as they had most of the bugs ironed out of the crossflows by that point – reliable CD ignition, no electric shift lowers, good, compact power trim, better idling carbs, etc. I have a 1984 Johnson 90 on my Starcraft, dressed to look like a 1976 to match the year of the boat.

EDIT: Scratch that last idea – 1971 Johnson V4s had a different cowling footprint, coming to a slight point in the rear, rather than the later, standardized cowling footprint that Johnson and Evinrude used for many years. Might be able to swap the belly pan, though, to make it work, if you really wanted to…