An affordable civil SST would require a purpose built engine, one that’s capable of supersonic flight but more optimized for efficiency than performance like military engines. AFAIK no engine manufacturer has one that fits the bill or is willing to spend billions to develop one just for Boom. Boom’s not gonna have an airliner ready to go anything soon, I think they’re trying to make a name for themselves to attract a partner with deep pockets like Airbus or some other established aerospace firm