Seems like it would be easy enough - you register your car, each model has an average and max decibel rating. Your congestion charge includes a factor for that - Harley's pay a bigger markup and EVs pay next to nothing for that particular thing.

It’s too easy to modify the exhaust on a motorcycle, so I don’t think this would be practical. Local laws here are very strict about noise levels, and vehicles are inspected regularly, yet still there are a lot of drivers with modified machines that can wake up the dead when revved.