If you took the overall airframe design of the F104 Starfighter, and replaced all the internals with modern tech, how competive with modern fighters would it be?

Not very because a big part of stealth is shape, and stealth is a big deal in 5th gen fighters.

Shape will also affect performance in ways I’m not even close to qualified to speak on. But the F104’s main assets, IIRC, were speed, climb rate, and ceiling. It wasn’t much of a turner or dogfighter. Now, that’s not a big deal because dogfighting is probably going the way of the dodo…but alot of 5th gen fighters employ various supermaneuverability features (thrust vectoring being one of them as you mention). It would be a question for an aerospace engineer on whether the F104 is suited to that sort of thing.

But why would you want to duplicate the shape of an earlier design?

@Entropy
“But why would you want to duplicate the shape of an earlier design?”

Because it looks good.

It has been said that if a really good pilot was in the F-104 Starfighter, he could be a close match to anyone flying one of the newest, most sophisticated aircraft. It was that good.

It seems to me the limits would be about size and stealth. But you could probably develop doctrines where that would be fine, depending on how you want to measure what’s good. Like, if you want a cheaper frame for modern missiles than an F35, you might be able to compete favorably in terms of cost per firepower airborne, and field more fighters and have less maintenance. But you’d be using more pilots, probably more fuel, and they’d be more vulnerable due to lack of stealth. On the other hand, if your missiles outrange your threats, stealth might not be so important.

A modern engine would allow supercruise. Modern electronics would make it compatible with modern fighters. But the wing sweeps and angles (esp. the tail), and exposed engine nozzle would be terrible for stealth. Same for all the doors and sheet metal joints – bad for stealth. New F-15 and F-16 have lower radar cross-sections. It would get it’s butt kicked by Beyond Visual Range missiles in no time.

It would be a disaster. All hell would break loose the first time that windows crashes.

The reason the plane grabs the eye is because you need only glance at it to appreciate that it is designed with a single factor in mind to the sacrifice of nearly all other considerations or practicalities. Put bluntly, the thing was built to haul ass and in the mid 50s when it originated there was nothing ANYWHERE in the field of production manned aircraft which might approach its pinnacle in achievement of that single goal. I don’t think a jet pilot exists who does not drool at sight of the thing. You must note this tendency in your own air force Loli, which almost certainly holds the record for flying the tricky to handle stub winged beauties into the ground.