I had the same problem as you - the white metal shafts on the Kort nozzles are elliptical, although the nozzles themselves are not too bad.
I have not been able to find any others which are the correct size though, so I have stuck with the originals.
I have filed down the shafts to fit inside a 5 mm brass tube and glued them with epoxy.
The 5 mm tube is a nice sliding fit inside a 6 mm brass tube which I have epoxied into the original drilled out plastic hull tubes supplied.
This has worked fine and I am quite satisfied with the result, especially as I have also replaced the tatty white metal rudder arms with home made brass ones.
Hope this helps,
Geoff