Another
HNoMS Olav Tryggvason or two would be a good option. Versatile ships, whose primary mission is mine-laying, but they could play different roles as well. (the defensive focus would be in keeping with Norway's neutrality stance).
Instead of the historic swap of dried codfish for Italian Caproni Ca.310 bombers, which proved useless; make the swap for equivalent value of
Italian MAS boats and torpedos. Those craft could have done serious damage to the Kriegsmarine in the narrow fjords and rocky island bays of Norway in 1940.
Bring forward the completion of the
HMoMS Alesund class of destroyers (improved and enlarged Sleipners)
Not strictly navy, but some coastal patrol planes with endurance and good performance in bad weather, to keep watch over the lengthy coastline would have helped. ( the Norwegians historically had
Nortrup A-17/Douglas 8A-5NS single-engined light bombers and
Northrup N-3PB single-engined floatplanes on order in 1940. They also had a handful of
Heinkel He-115 twin-engined floatplanes