Could a modified J58 jet engine be used commercially?
According to Wikipedia the J58 is comparable to the Rolls-Royce/Snecma Olympus 593 that powered the Concorde, so yes it's possible.

Will it be efficient? The Olympus 593 was as efficient as can be, even by today's standards:


  The Concorde cruised at Mach 2.05 with its engines giving a SFC of 1.195 lb/(lbf·h); this is equivalent to a SFC of 0.51 lb/(lbf·h) for an aircraft flying at Mach 0.85, which would be better than even modern engines; the Olympus 593 was the world's most efficient jet engine. However, Concorde ultimately has a heavier airframe and, due to being supersonic, is less aerodynamically efficient, i.e., the lift to drag ratio is far lower. In general the total fuel burn of a complete aircraft is of far more importance to the customer.


It's not just the engine that makes an aircraft efficient, as the paragraph above highlights. The amount of fuel the Concorde burnt for 100 passengers is the same for 700 passengers on two Jumbo Jets.

As I highlighted here, even if fuel is cheap, or the ticket price covers the fuel cost, propulsive efficiency isn't the whole story. The Concorde wasn't retired because of fuel. I'm mentioning the Concorde because of its very efficient engines and supersonic regime.