Tear in fabric seat
The easiest repair I know of is to use iron on mending glue from the reverse side. This would require that you remove the cover from the seat to repair it. If you can do this, go for it. You are going to make a sandwich out of your seat fabric, a layer of heat meltable glue and a patch of some other fabric. The sandwich will be on the underside of your seat cover. The glue will melt into the fabric and hold the cut edges together with no sewing or darning. You'll get a butt join that if you line up neatly, will be very difficult to see.
Don't just run out and get the patches that strengthen the knees of a kid's jeans. They are the wrong combination of glue and fabric for what you want. Go to JoAnns and get midweight Wonder Under and a small piece of sturdy fabric that matches in color. Follow the directions and don't use too hot of an iron. Give yourself about 1 to 2" of patch around the tear. For a 1/2 inch tear, your patch should be an oval no bigger than 2" by 3". If you look pitiful enough, they might simply cut you a sample swatch and not charge you for the fabric.
If you can't take the seat off, you might still be able to use the mending glue if you have a tacking iron. You create the glue and backing fabric 'polkadot', feed it through the tear, line it up and then mash the tacking iron on to melt the glue. Heat guns will not work, I've tried.
PM me for more details.
Jean