Why would NK buy rocket motors?
They are experts at stealing plans for anything. With plans, rocket motors can be made by many machine shops.
If you don’t build your own, you are then dependent on your supplier to replace the ones used in testing.
Very tricky supply lines. How do you create improvements if you don’t build them yourself?
One of the points Elleman report made was that N. Korea has no history of experimenting with this type of engine before. He claims that Musudan and Hwasung rocket are completely different.