Karl,
The Motobecane mixte frames w/1020 tubing first appeared in 1974 and last appeared in 1977, when 2040 tubing replaced 1020......"for those who demand the most but with a conservative budget".
None of the 1020 mixtes however had Mafac brakes (at least according to Motobecane's literature) nor Stronglight 49D cranksets!! In 1974 the Mirage model featured 1020 tubing, Weinmann 999 center-pulls, Nervar steel cranksets, Normandy QR hubs and steel Weinmann rims.
The 1975 Super Mirage came with alloy rims and Atom alloy pedals, Tourney Luxe alloy crankset and SR stem.
Pivo bars/stems were never "specd" on 1020 tubed frames, and by the way, the wheel size was 27" on all the 1020 models (you did not specify the size of your rims).
It's obvious the bicycle has been upgraded, and I certainly wouldn't feel bad about "pirating" parts off of a mixte. For an exact dating, what is the color?
Hope this helps without muddying up things too much!
Take care,
Chuck Brooks
Malta, NY