Out all things a supersonic aircraft needs, an engine is the roadblock bigger than everything else combined.

Uncounted supersonic aircraft startups went bust trying to make own engines (and that with making turbojets actually being easier than modern turbofans)

For everything else, it's sixties-seventies materials, and technology.

In fact, it's much easier for example to get steel, and aluminium alloys withstanding 200C°-300C° than in sixties.

Strong actuators for control surfaces are both better, and easier to make now because of aircraft electrification.

Internal structure can be made easier with composites today than with complex forged parts 60 years ago.

And of course, supersonic aerodynamics is now much easier with computers.