If I build it myself, then I don't know if I built it correctly. If it doesn't work, it could be me who takes the blame.
I would need them to send me an engine that they've confirm works, so that when I test it, I know ahead of time that it works perfectly, and that my test cannot go wrong.
When it comes to science and technology, we cannot give people a break. For science!
Each hypothesis must be proven. That is the glory of science, and it is an honor to be asked to prove a hypothesis. I still hope that they honor my request to test a pre-built, confirmed-to-work engine.
But they have not, so I declare this machine fraud.