NGK plugs are one of the brands that do well in beemers. So don't change because of this. It's not the plug's fault.
As you said, the bike sat for 26 years, so I'm assuming corrosion is the issue here. It would take patience on your part, but this is the easiest and cheapest way to resolve this one:
Heat/Cold cycles
Penetrating oil
Back and forth motion
Vibration
Heat up the spark plug remains with a propane torch.
Douse with penetrating fluid
If you can wedge something into the stub remaining - like one of those square sided easy outs - work it CW and CCW - putting just a small bit of force on it.
Let it cool, or even super cool the plug remains to shrink it while the aluminum of the head is expanded.
Back and forth - tap on it to set up vibrations. One metal will vibrate at a different frequency than the other, thus rejecting/breaking the bond.
More penetrating fluid.
Do these things over and over - perhaps only once a day, or for a half hour or longer.
The heating/cooling cycles will allow enough of a gap to develop which will allow the penetrating fluid's ingress. After three or four days, perhaps a week, of this, it'll suddenly move just a tiny bit. Turn it the other direction. More fluid. Back and forth and it'll eventually spin right out.
Once it's out, be prepared to flush the cylinder. I've used a small hose taped to the vacuum cleaner to get inside the cylinder and flush any small remaining bits.
Don't get the idea that because this happened, the threads are buggered. Not necessarily so! Take your time, be patient, and all will work out fine.
