Even using the same roster of ships as they historically used, have the Norwegian leadership issue an earlier and clear order to shoot at any unidentified warship coming up the fjords on April 9, 1940. It's still going to go badly for the Norwegians that day, but any damage done to the invasion fleets, particularly in Narvik, Bergen, and Trondheim would really crimp German plans. The Germans needed to keep to tight timetables for success.
Another thought: instead of trading cod for the useless Caproni Ca.310 bombers; trade with the Italians for some MAS boats with the accompanying torpedos. They might have been very effective in the close confines of the fjords where bigger ships had limited room for manuever and the little boats had some rocky islands for cover.