Reminds me of the pre war bikes I used to ride in the 60's. I rarely had an exhaust pipe and you could see the exhaust valve opening and closing.
Looks very similar to the RR Aircraft Merlin engines I used to take to bits in the scrapyards. They were 27L 4 valve per cylinder with single overhead camshaft. RR also made the Meteor, similar to the Merlin, for use in a tank. Both petrol.