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John,
if the tear is a clean cut I would glue a piece of unprimed canvas on the back side, using acrylic glue.
Cut a piece of canvas 1" wider and longer than the tear and glue it generously and place it on the backside of the picture. Then support the backside with something flat so that you can apply pressure on the front side using your fingers. When the tear is put into place, like a doctor would fix a wound, you put a piece of plastic over the cut and a book or something until the glue is dry.
Then you can fill in the missing paint with a small brush.
About cleaning, you could try with some mild soap water and a stiff bristle brush to see if it is only dust and smoke that needs to be removed.
You clean s small area at a time and wash with clean water on cotton so that the surface is not wet for long.
If it is varnish that has yellowed you will have to contact a pro.
When the canvas is clean you can varnish to freshen the colors.
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