Correct me if I am wrong, didn't the early XKE's have the straight 6 cylinder? They weren't known for their horsepower to weight ratio. Maybe the Jag could use the 250 GT engine.
It would be tough to dig one of those up at the local boneyard.
I get not wanting to butcher a classic. An early Jag is most definitely a classic, not quite on par with the 250 GT though.
The place I worked for 30-some years did the engine machine work for Group 44, the factory Jaguar team in the late 70's to mid 80's. The owner had one of those later XKE's but I could swear it was a 12 cylinder. He also had a XJS that had a 420 cubic inch Chevy small block. It was wicked fast compared to the original 12 cyl.