I'm not sure of the Canadian laws with regards to parts kits but in the US it seems to be a very murky grey area.
EBay has a vendor selling partially-complete "display" receivers and, as far as I've been able to tell, these are legal to possess.
However, if you complete the last few drill holes, you now possess an illegal machine gun. It doesn't matter that all you have is a hollow tube without any internal parts, etc.
US laws dictates that the receiver is not a gun uless it's more than 75-80% complete -- after which it's not only deemed to be a firearm but it's also deemed to be a CLass III machine gun. Again, one needn't even have the barrel, springs, mags, etc -- just the metal tube.
I haven't a clue as to how one could argue the percentage of the reeciver that's been completed, if nabbed in the field by law enforcement officials.
I ended up torching the receiver on my Mk III and then welding the two halves back together again -- it's now deemed to be demilled and is, once again, just parts.
Best bet is to find a legally converted semi-auto sten or one that has legal paperwork.
The last alternative is a sten kit with a solid aluminum receiver. It will look fine for display but they're useless for any type of reenactment use (i.e. firing blanks, etc)
I'd be curious to hear of anyone who's using a sten for reenactments. As far as I've been able to determine, there's not way to make an operable blanks-only sten that is legal to possess -- this is for the U.S.