I can understand the desire to change out the Simplex - I had a chain jam in one the other day and the whole thing kind of "exploded".

I just went to my parts bin and chose the cleanest Shimano SIS off of some big box bike I stripped and had it adjusted and running in about half an hour - though interestingly the Shimano seems to need more cable travel to reach all the gears than the Simplex did. I am still using the Simplex friction shifters. Shift much better.

I should think, that for a quick and dirty period correct all metal look, an old Shimano Tourney would look good, and I would expect an improvement in action over any of the later plastic Simplex units, eh? I seem I see those Shimano Tourneys on about half of the lower end bikes from "the correct" period - the other half have ... something else. I have a couple in my life now that came on bikes I got for 10 bucks at the ReStore.

Then - you have a good looking rider while you shop for a period correct upgrade unit - or modern unit that looks right ... if you must ....