I don't know about compatibility issues. Does an 80s Puegeot have French threaded parts still? If they're English treaded, other than a possible need to spread the rear triangle to 130mm from whatever it is to accommodate 8+ speed wheel hubs and cassettes, there won't be any compatibility issues, just cost of new parts and every "modern" component can be fitted.
If French treaded, both headsets and bottom brackets are available from places, Velo Orange for one. A 1" threaded headset and quill stem can be easily changed to modern threadless stems if needed - or left as is with the current handlebars or any number of good 1" handlebars that are still available.
A new French bottom bracket will probably be square taper, but there are plenty good cranksets of various gearing, that are square taper and the "disadvantages" of the older bottom brackets and cranksets are generally not a factor at all for the recreational cyclist. Or even racers in fairly recent times past.
If the BB is English treaded, practically any current crank and its threaded bottom bracket can be fitted. If the current BB is ok and the gearing of the current crankset works for the rider, no need to change them at all. They will work fine with new shifters and derailleur.
I don't mean to trivialize about the cost of new parts. A complete upgrade, is a complete upgrade whether it's a 20 year old bike or a 40 year old bike, the cost would be the same. I was addressing compatibility which might or might not need to be addressed, but it is easily addressed either way.
As to the original post - adapting downtube shifter bosses to cable stops is easy, they're readily available.
Last edited by Camilo; 05-26-22 at 06:32 PM.