Bristol used/reproduced the BMW engines under license from AFN. The technology was pre war, BMW 328 for example. Bristol only started to manufacture cars after WW2. The Ace had 3 engines during it's lifespan, AC's own engine, the BMW sired Bristol engine and then the Ford straight six, which with triple carbs and a Mays cylinder head was the fastest of them all. Great work on the front end JC.
Local modification I'd imagine JC. There was a category in the UK back in the 70's, all sorts of thing were allowed, Jag XK 150 with wide slicks and a spoiler, Beetle with F1 suspension etc. It was called Modsports, so it could be a left over from then, unrestored and slightly modded. Just a guess.
I was glad to see the square headlights too, you need them for the Alan Mann racer.,nice red and gold paint job and you're done. But I did see a review (that I can't find again..) saying that it looks difficult to lower it down to Tarmac spec.