You may wish to consider dye penetrant as a viable alternative to "magnaflux". However, cracks in Norton crankshafts are rare. And should they develop, catastrophic failure is soon to follow. So, if your crank is one piece, chances are it has no cracks.
Notably, reputable sources recommended that you replace all the studs and nuts on your crankshaft, as long as it’s out of the engine.
You may find a friendly workshop at a local small airport, they usually have magnaflux capability for aircraft maintenance and inspection work, they may also be able to help you find a good machine shop. Good luck, James.
I have hesitated to mention this shop because I haven't used them, but at least it's a start.
Q & E is in Anaheim. They are a motorcycle machine shop specialists. Their shop is full of cranks, barrels and heads. I spoke to the young man there and mentioned Norton and he lit up. He went on to explain how this shop, specifically his dad used to do the big engine cranks and more for HPI. He also mentioned his dad is still active in the shop. HPI used to be less than 10 miles from this shop so it is believeable. Besides, who would even know about HPI.
When it comes time for me to do some machine work, I am going there, but currently I have no experience so I cannot comment.
Q & E
1140 N. Kraemer Bl.
HPI was a hot rod shop that specialized in very fast Nortons. They were right here in my back yard in Stanton, CA. They were among the first to offer 900+ kits and the Drouin supercharger.
The irony is I never had any experience with them. In the 70's I was told of this shop that could make my Norton scream, even supercharge it if I wanted. I never looked them up because I was just happy to keep my Norton on the road at the time.
George has left us now and HPI is forever gone.
I am sure there are others out there who know more than I and I would be most interested in hearing about this fabeled shop.
For instance, I know Ron Woods (Costa Mesa) and C.R. Axtell (Long Beach / Torrance) were aligned, where does HPI fit in this, if at all. Costa Mesa and Stanton are less than 10 mi apart.