If your engine used reed valves as standard or if you can prove it was a common modification before 1967 then the are allowed (same goes for disc valves). Also If your engine was piston port already you can convert to reed valves if you can do it "without changing the external appearance of the engine".

How strictly this rule is enforced seems to vary massively, but basically if there are 20 bikes that look different to yours because you've changed something then it's not allowed. If 20 bikes look different to yours because they've changed something and yours is original then theirs will be allowed. See 95bore manx nortons with 2 less fins on the barrels as a good example of this.