It's true that a lot of aircraft development has occurred using substitute engines while the final engines where still being developed. The problem for Boom isn't getting airborne. It's convincing one of the few outfits capable of this to put billions of dollars into a clean sheet engine design.
Klimov RD-33's (the Mig-29 engine) are not super-cruise engines and aren't capable of demonstrating the intent of the Boom design. And frankly I think even a retrofit like that is beyond Boom's means.
