You can either a) rebuild the Concord or b) use modern technology.
If you use modern technology, its not a Concord anymore.
And you can't magically go Mach 3 just because you say its 'modern'. What existing engine can do that? And even if you had an engine, a Concord will not do that anyway.
So really you are talking about developing a whole new plane. And that's gone cost 10-20 billion $ and including the engine like quite a bit more.
You can either a) rebuild the Concord or b) use modern technology.
If you use modern technology, its not a Concord anymore.
And you can't magically go Mach 3 just because you say its 'modern'. What existing engine can do that? And even if you had an engine, a Concord will not do that anyway.
So really you are talking about developing a whole new plane. And that's gone cost 10-20 billion $ and including the engine like quite a bit more.