I don’t think your old o-rings were “squashed badly.” The original o-rings supplied by the factory had a square profile with a semi-circular lip around one side. These were possibly a custom-made item for the ends of the early Jaguar cam covers. I have three of these unique o-rings in my collection of spare parts that I removed from early XK engines.
The spare parts suppliers now provide generic round-profile o-rings for this application, which probably don’t seal as well as the originals. I suspect a generic square-profile o-ring would seal better, but I’ve never tried to find the best size.
Here in the U.S. we have McMaster Carr who claims they can make custom o-rings for special applications. One of my “to do projects” is to send them several of these original square-profile o-rings supplied by Jaguar to see if they can reproduce them at a decent cost.