They built Dubai in record time with money flowing freely.
Yes, but Dubai is basically a city. There is a lot of territory in North Korea. Pyongyang will be modernized quite quickly, but outside the capital, there are a lot of smaller cities that will take time to upgrade.
How many contracts will US companies get? That is the question. Will it all be Chinese, Russian and South Korean? There could be some good long-term contracts for US ex-pats.
Dubai had a tiny population - its "population" today is 80% expatriates.
It is built on cheap labor imported from South Asia merged with expensive Western knowledge - there's no way that could be duplicated in Nork
The norks are underdeveloped and famished due to long famines. They will need a generation to get over this malnutrition.