Real tractors are big vehicles - it's not hard to find somewhere to mount a small plate. All road-going tractors in the UK will have both front and rear plates - you can see it in this photo just below the cab:
That's not quite my point - you asked where would you mount a rear plate on a tractor, to which I answered that there are lots of places. If you're talking about the fact that plates can be obscured by large trailers etc then local laws will dictate that there is a plate on the trailer. I guess plates covered by attachments (ploughs etc) are a slightly different case and may obscure the plate.
In Germany, any motor vehicle which can go faster than 6kph has to carry some kind of license plate, depending on the category of the vehicle. Even eScooters do. For those, and for slow motor bikes, this is issued by the insurance company, as insurance is also mandatory for any motor vehicle, for most others it is issued by the transport authority.
Where would you stick the license plate on a tractor?