One of the things which is happening with historic racing is a lot of the bikes are actually retros. I would never stop a retro from racing. The best rules are no rules. All that needs to be done is define the race classes by the type of bike and it's engine capacity with only ONE cut-off date. A Paul Smart Replica Ducati should be able to race against the Irving Vincent. A Molnar Manx and a Walmsley G50 are both retros. And so are the MV3s that ran at Goodwood Revival this year. There are only four types of motorcycles - two-strokes and three four-strokes - singles, twins and multis. Capacity classes can be combined.