if that were mine (and it fit me) I'd definitely keep it! If you want to sell off the stem you can probably find an eager Colnago owner who wants it, looks like an ITM, true? Also would not recommend this but the water bottle might also be collectible to someone, but probably best to keep it with that frame.
Share some more pix of this!
BTW, that Cinelli IC BB shell has "FRANCE" cast into it, so I'd bet that indicates it's made for Metric tubing (but measure the OD of main tubes to confirm that) as to whether it's metric Columbus or metric Reynolds perhaps the fork steerer can help with a clue. 26.6 if correct MIGHT indicate a light/thin-walled set perhaps like 531SL...perhaps...