May need a different stem. "Old" flat bar is 25.4 mm clamp diameter. "Old" drops are often 26.0 mm.
And sticking a 26.0 bar in a 25.4 mm clamp is generally frowned upon due to concerns about crimping becoming a stress riser and cause of failure.

And no sooner had they agreed on 31.8 mm bar clamp diameter, when someone came up with a 35 mm diameter...