There is nothing special about that webbing you can buy it at the hardware store. Then you take either an industrial sewing machine or an awl and sew it together or get someone to do it for you. You might want to source the webbing yourself before you take it to a shoe shop or sailmaker. I usually just use an awl and sew it together myself.
doesn't matter too much if it's nylon or polyester, the webbing used has a strength of a ton or so at that thickness, all that matters is when it gets worn down too much it can fail.
Once it is properly repaired it should be stronger than brand new as typically the harness makers typicall use some v96 polyester thread or the equivalent to make it initially and the repair will have much stronger thread that will wear longer.
Most of the wear from the harness comes from the spreader hook rising up and falling down and then the webbing wears at the sides of the spreader bar. I get that a lot because I like to wear my harness pretty loose. I think the ride engine webbing lasts a bit longer because of the way they have it attached to the spreader hook, it doesn't move around as much. I think if you wear your harness really tight the webbing doesn't wear as much but I don't like a tight harness.
polyester has better uv resistance, but I usually get nylon webbing anyway.