Try loosening the set screw and moving the clapper one way or the other and see if the ringer works better. The position of the clapper will affect the frequency at which the ringer will work, much like a pendulum on a clock.
Finding a replacement ringer should not be terribly difficult in Montreal. If you look around, you will likely find a junk phone (500, 554, 2500, 2554 and the like) at a yard sale or thrift shop which will have a usable ringer in it. Also, I would expect that someone here on the forum has a spare that they could supply for a reasonable price. There are a number of members in eastern Canada.
Larry