If you're flying Air Canada, you have a better chance, I think, but I wouldn't feel comfortable trying it. They are good with wheelchairs, though: Last Feb, DS broke his leg right before we were supposed to leave for WDW. He was in a full leg plaster cast, and wasn't strong enough to lift the thing, so we had him in a wheelchair. As we were wheeling him in the direction of Express checkin, and AC rep asked us if we were checking in for Air Canada. We said yes, and he headed us towards the special services counter. They checked us in, handled our luggage, and wheeled DS through customs and to the gate, expediting the process the whole way. They are really good about helping people with special needs. The way back was similarly expedited, but it still took us about 35 minutes (no bus out the back, though, just wheeled through the front, maybe things are different if you have a deadline).
I have to say, 50 minutes between flights, especially when one is international and the other domestic, is not a good window under ideal circumstances. When you think about how people react when a plane lands, it won't be possible to take a wheelchair off the plane first, so you're looking at about 15 minutes on the ground before you get off the plane. Immigration can be quite swift in YYZ, but that's not the sort of thing you want to bank on. Then you have to grab your luggage, and go through customs. The fact that you have a connecting flight won't make you exempt from a random check, so that could happen as well.
I know this isn't what you want to hear, but can you book a later connecting flight?
Good Luck,
Mort.