I'm a little fuzzy on the details now, but Transport Canada has a law on the books regarding Unimogs and I believe it says that any Unimog produced between 1996 and 2002 ( I maybe a little bit off) cannot be imported into Canada, not even for off-road use.
Calling Transport Canada may be useful, but I would tend to think that unless you speak to the guy who deals with Unimog (apparently there is someone in Transport Canada who is the Unimog expert) you'll get bad or incomplete information.
MB Canada has sold Unimogs through various channels over the years but I am not aware of them selling any in the 90's although they did sell MB Trac's here for a limited number of year in the late 80's and maybe into the early 90's.
The bottom line is that there is a very good chance someone could import a newer Unimog into Canada and have it sneak under the wire and get a plate from one of the provincial Ministries of Transportation, but if Transport Canada were to ever catch them the result would not be pretty. If they were lucky they would have to export the machine, worst case it would be sent to the crusher and the owner would have to pay some fines.
Please keep in mind that this is all second hand info from a couple years back so I could be completely off base. The person looking to import the machine will have to do some serious digging and spend many hours on the phone trying to find the right person with the right information within Transport Canada.