Hello from Colorado and welcome to the Airstream community. Dexter hasn't made ordering replacement axles very easy. I recommend you find an Airstream axle "expert" to place the order for you. Maybe someone can recommend someone there in central Canada.
Your trailer was designed with a "gross vehicle weight rating". It is on the VIN tag likely on the front street side of the trailer. You can remodel your trailer with a masonry fireplace, granite countertops and a tile floor, but the frame, axles and body were designed for the gross vehicle weight rating. I wouldn't exceed it.
Let's pretend it says 7600 pounds. You will have about 800 pounds of trailer weight on the tongue, so we will subtract that yielding 6800 pounds on the axles. You have two axles, each carry about half that weight. So you would need axles built for 3400 pounds. That might be a good weight rating axle to order. You can crawl under your trailer and try to read the dirty, rusty, painted over tag on the axles themselves.
You mentioned a "lift kit". Some people do this because they tow their trailer up old mining roads in the mountains, or they live around roads with steep approaches. These situations are hard on the trailer. Most of us don't need any more ground clearance than Airstream originally designed for. I don't recommend increasing an Airstream's ground clearance. Others will disagree with me.
I do recommend ordering bearings and 12" drum brakes with your axles. And new shocks too. The welded on shock mounting brackets are also an important specification for new axles.
Here is a photo of my original axles under my Overlander. You can see how the "starting" angle is no longer 22 degrees down, but the swing arms are now horizontal. Not good. This reduces ground clearance and signifies the rubber suspension rods are hard as a hockey puck. I'll mount new axles this spring.
Others more knowledgeable than me will surely help out here.