As far as I know, the Norvel smalls (.049/.061) use a 3mm prop screw. Mine are buried in the hobby closet right now. Can you confirm this?
Really, your current/best/lightest option is to take the Cox hub to your favorite machinist buddy and have him replicate the hub, but drill the center out for the 3mm prop shaft. Note I didn't say cheapest.
Cross-referencing a drill/tap chart shows #5-40 screw holes take a #38 drill bit, 3mm takes a #39 bit. If you have a Cox prop hub, and have access to a lathe or drill press, then use the 5-40 tap to line up/mount the hub with respect to the chuck. Switch to the tap required for the 3mm prop screw, and run it through, turning the chuck by hand.
I recommend NOT free-handing this one because the metric tap will want to follow the imperial thread for a few turns, and this will cause the tap to wander off center/angled. You won't have much meat on the threads either, because they will be basically cut into the #5-40 threads.
I also like .061 engines, and have a small smattering of the APs. They have a 4mm prop screw. I purchased a drill bit and tap to make a Cox spinner hub fit the 4mm thread. I doubt I left much of the #5 threads, and the tap cleaned out what was left while creating decent, usable threads in the body of the hub...that's how close the clean-out cut was!
If you can dig the Norvel crank out of the block, and drill/tap for the AP/Norvel .074 prop screw, then you would be essentially doing what I did to the AP. Bit that is twice the work.
Oh, and I really like having the bigger spinner in place. It sure beats looking at the puny spinner that came on the engine!