If your tires are bias nylon tires weather cracking doesn't really hurt them. It was common for farm trucks to run tires 30 to 50 years. Tubes lose their elasticity with age. Many flats are caused by tubes folding and starting to leak at the fold. Tire dressing can make old tires look better and deep cracks can be filled with rubber cement. Used tires probably won't be any better than what you have. But you probably need new tubes and good flaps are also very important. They keep the tube from rubbing on an old rim. New flaps can still be purchased.These guys still have flaps and sell used military tires.

https://www.genscotire.com/index.html

I'm not sure if shipping to Canada is an option. I am 78 years old and they have been around for as long as I can remember.