If you watch that video, most of the bikes are four stroke twins or triples. There are no four cylinder bikes and only a couple of two strokes. It is the Mike Hailwood Trophy Race and it is held at Goodwood. The Poms are not stupid.
I don't have a problem with Rex's 1100cc CB750s in period 4. But the Irving Vincent is ridiculous. The capacity limit for historic racing should be 1000cc.