My early 1970's Sears bike has bike but the frame, seat post and calipers that is period correct.

Cotterless crank versus the original cottered (actually it has a triple installed now that I will replace this weekend with a 50/34 compact built on a Sugino XT crank I have).

Alloy rimmed wheels instead of steel.

6 speed freewheel versus 5 speed.

Indexed stem shifters versus non-indexed.

Nitto Technomic stem and alloy handlebar with aero brake levers

Cork wrap versus plastic.

RD is currently a Deore, but since I am eliminating the triple, I may switch to a Shimano 600 instead of Shimano Eagle.

FD is probably destined to be a low end Campagnolo instead of the Shimano Lark... If not I have a Shimano 600 I can use.

Seat is a Brooks instead of the original plastic seat.

Of course, this is on a bike that wouldn't be worth much if it were in brand new showroom condition, but even so, in general I think that upgrading whatever you want to upgrade is the right choice for you.