John, we find that if an old sidecar has a number on it when we buy it, it's not a bad idea to try to keep it with the outfit-kinda like a boat name-bad luck to change it. It might also help to keep track of them. Prob sounds like shit but it keeps with the 'history' of it all.
As far as personal plates go-if you have a 'new' bike, it's good to put your stamp on it especially if you have some 'history' which it appears you have
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If it's the same as one you raced years ago-put that number on it. Good for good memories.