Hermeus is starting with the engine [1].
I like their explanation of the choice from a recent tour:
(Youtube transcript)
"So these are out of production, the J 85s. So we didn't work with GE at all. It was all just us working with, we were really working with the maintenance, repair and overhaul shops for them. That's really where the expertise and knowledge lies. These engines were, I think originally designed in the fifties. There's not a lot of electronics on board. There's no firmware we have to work through.
And really, it's a pretty elegant but hydro mechanical system for all the controls. So really it was about understanding the configuration of it and you can kind of chase down all the different tubes and everything to understand how it works. And then there's a suite of documentation out there. So it was really on us to learn how it worked."
I suspect they used these just to get the XB-1 airborne, making progress while they find a better engine.
Besides, most countries couldn't build a modern jet engine even if they had the exact engineering drawings of an existing one available. From fashioning single-crystal turbine blades to establishing a supply chain capable of the quality controls needed, the amount of hours spent is unreal.