It doesn`t make sense to get a historic plate for a MC in germany, because the tax is higher! A 125 is tax free and for the historic vehicle ~46 EUR. Maybe for insurance reasons, but that differs too much from company to company. Insurance should around 100 EUR for normal registrated bikes and maybe 70 on h-plate. I say around! it really differs alot. Anyway you can keep your plate, but your are not allowed to display it on the bike anymore. Your documents will be stamped void.