Actually, if it was a five speed, you might want to measure the rear dropout spacing before running off and buying a new wheelset. Older droputs, in particular five speed dropouts, were spaced at 120mm. An eight speed dropout would be 130mm, most six speed ones are 125mm or 126mm. To go from 120 to 130 would likely require cold setting the dropouts (i.e. bending them) which is really one of those jobs you want to have someone who knows what they're doing do. Check that spacing. Older freewheel hubs can be spaced down from 126 to 120 mm easily enough. If you wanna cold set the frame yourself, there are primitive instructions on Sheldon Brown's site, but I'd by a couple of old junkers to do this on before risking a Colnago.

Aside from this small problem, however, you certainly can equip the bike with newer componentry. Unless you're married to having the bike in exact period condition.