I have a 1975 Masi Gran Criterium. It was made in Italy in Masi's workshop in 1974. I'm the original, and only owner. In 1981 I sent it to Masi America and had it repainted. I had them put top tube cable guides on it (originally had the SS bolt-on campy cable clips) and added a 2nd water bottle boss set. When I got it back, I put new SR Campy parts on it. I put a SR headset and rear derailleur. The crank set is the original NR set. I put the Super Leggero pedals (black alloy cages) on it. After rebuilding it, I only rode it about 50 miles. It's been hanging up in one of our local bike shops as a display for years but am considering selling it.
Since it's not an original spec bike, I assume it's worth less than if it had been kept original, but as a perfect piece, it looks as good as NOS equipment.
A few things. Images help. Things that will effect its value are:
The fork crown type
The stamps on the bottom bracket
The seat lug style
The transfer set selected by the painter, this bike should have the two color down tube graphics, Masi on one color and Gran Criterium in another, like yellow and white.
The updates take away from the value
The repaint does too, especially if it uses the later transfers.
Faliero or Alberto signature, which should be on the non drive side.
Things that can raise the value.
"Big" bottom bracket windows
Milled rear dropouts.
Twin plate fork crown
Long tang on the seat lug down the front of the seat tube.
Original invoice from the Vigorelli
Letters, or images at the time of sale at the Vigorelli.
The original pump and or waterbottle