There's a lot of support required to develop a rocket engine, and just having plans or a single example is not really sufficient.
The classic case was the Russian NK-33 engine. A US company had 100 of them, obtained from the manufacturer. But the engines had been stored for 30 years in a shed in Russia, and the metal components degraded, without the manufacturer understanding the issue.
The original designers worked at another company, but they were not consulted in the engine purchase deal.
So, eventually, the engine decided to explode upon launch.
The Russians still use the engine, but only those acquired from the designer, not those stored by the manufacturer.