I would use 'mounting tape' - double-sided sticky foam tape - just like they use for sticking number plates on cars today.
A couple of shaped wooden blocks would be needed behind the plate to get it to stand roughly vertical, with mounting tape between the blocks and body and again between the blocks and plate. The blocks shouldn't be visible behind the plate.
And I doubt any policeman knows the Construction and Use regulations from back then, so a small motorcycle number plate could be used to make the job easier - I assume the alloy ones are available in small straight configuration, though they aren't generally used on motorbikes like that.