The halogen says 14v 35w ... in fact you can still buy the bulbs :
https://www.eevblog.com/forum/projects/halogen-vs-led/You can assume that's 14v AC and I guess it's supposed to be used with lamps or things that use a dumb 12v AC transformer inside to provide isolation from mains.
You could use LEDs but first you must convert the AC to DC using a diode (cheap but led would be off half the time, so you'd get flicker) or a bridge rectifier (for full wave rectification, less flicker)
Then you'd add a small capacitor to smooth the rectifier voltage and then you'd want to limit the current going through the led otherwise it would burn up.
Now you get a LED with a forward voltage less than your average DC input and you need to limit current going through it otherwise you'll damage it. Either use a plain simple resistor or use a led driver
