On the purely practical side : The NG
engines are digital, the 741 cockpit/controls are string and cams. So you need a new cockpit. And then the wiring throughout the aircraft needs to change to suit, and that ain't cheap.
Obviously, you don't want to suffocate the passengers, so cabin air supply is required. The NG
engine don't supply that, so you need another new system...
The thrust levels for the NG
engine are higher that the example 741 engines, so what do you do? Operate the same engine at lower speed/thrust ? Then you carrying around a heavier engine than you need. Redesign the engine to get the weight out ? You'll then need a new FBO test. Use that extra thrust ? The aircraft just wasn't designed for it.
Really, you'll just spending loads of money to end up with an aircraft that's probably no better than more recent (743?) aircraft, available cheap from the deserts.
To use a technical phraseology "It's no good trying to polish a turd".