If they work or not on modern day radar, here in the Netherlands equipment warning you for radra control is illegal. Costs 240 euro's fine and the confiscate the equipment.
And oh, some police cars have a radar detector-detector on board!
"If" you found one you "could" modify it with modern components. But why? I would use the Fair Warning as is for nostalgic reasons. Then integrate a modern day one that will detect K and KA. Laser is a nice bit of false security, because by the time your detector gives you a laser warning you are dead in the water. I use a Valentine one and drive a freeway to work on hour everyday. Its saved my ass on KA, however I just got hit with laser the other day, lucky I wasn't speeding. Within time all law enforcement will update to laser, its just expensive right now.