SGI used to let you plate a bike that was designated by CMVSS as a "Restricted Use Motorcycle". These will have the letters "RUM" stamped into the sticker usually located on the steering stem on the frame. To do it officially, you had to add all of the street hardware, tires, muffler, reflectors, etc., and then technically you had to change the suspension components to components from a street legal bike. This made plating something like a CRF450X difficult, but something like a DRZ400 E model (the non-street version) was pretty straightforward because it came with the same suspension components as the street legal DRZ400ES. Many KTM non-legal models also have similar suspensions to the 500 EXC and 350 EXC - I was looking at a 300 XCW as a neat project street bike at one point.
However, I think they have recently quit doing this. See the footnote at the bottom of this web page:
http://www.sgi.sk.ca/individuals/regist ... index.html.
If you decide you want to try the unofficial version (I don't condone this, but some people do it) you could just try setting it up to comply with all of the things on this web page:
http://www.sgi.sk.ca/individuals/regist ... tions.html. Then go to SGI and ask them to plate it for you - just give them the VIN and the make and model. They may require an inspection, or they may flat out tell you that they can't plate that bike, but people have done it.
** Keep in mind if you do this you may not be covered by SGI in event of an accident!** They have every right to refuse coverage if you have misrepresented anything, or if you have registered a vehicle type that they don't allow to be registered. You could be paying for the rest of your life if you injure someone else while riding your bike if you don't have liability insurance.