"Max weight"? Boy, that's a tough one...must be a Canadian thing, I've never heard of that. You might look on the axle for a plate...but being a vintage trailer, it may not have one.
You could load up the trailer and take it to a scale...but I'm sure that's not what they are looking for.
You might have better luck asking on a Spartan-specific site - I don't think there are very many owners here to respond to that kind of technical trailer-specific question. I do know there is a Facebook page of Spartanette owners - you may want to check-in there too as well as with TinCanTourists.
This might help too: Spartan Production Information