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Can you not just make the plane smaller and lighter and more aerodynamic and use a larger engine? You may also be able to show that current engines can operate at higher energy levels for longer?

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Doesn't work like that. They want to use a J28D engine modified for super sonic flight. That engine was designed to power MD80's.

Its the intake design that makes the difference for supersonic performance.

JT8D [0] is based on the J52 engine that powered the A-4 Skyhawk, and a related design is the RM8 [1] that powered the M2.0+ SAAB Viggen.[2]

[0] https://en.wikipedia.org/wiki/Pratt_%26_Whitney_JT8D

[1] https://en.wikipedia.org/wiki/Volvo_RM8

[2] https://en.wikipedia.org/wiki/Saab_37_Viggen

Engine is generally optimised for max efficiency at a cruising speed. A airliner engine is optimised for transonic cruising, you can't just half the plane and double the speed.

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