Charles,
First, I would agree with Harry, a Mini Stretcher would make this job very simple, very simple.
Since you do not have one you can trim it and put a small insert, somewhat of a saddle. Grafting would also work if you still have the secondary in place, which you did state was still good, this would be a viable repair since it is on the side of the seam and not in the middle.
A couple of these type repairs could be made but it goes back to the mini stretcher, with that tool, a glue gun and a kool
glide you can pull the end together and make it look great in short order.
If I were to try to graft it, I would probably trim as much bad stuff away and then form a piece to fit but I would not take off the secondary, I would 'graft' it with the secondary still on the donor piece and lock it in place and then trim the top off. This way you keep a lot of the stability of the donor and still never break the backing open at the seam and let what is stable stay stable. This is a pretty good repair for this as well but you have to make some anchors through the backing for the donor piece to hold onto. This is done with a glue gun.
Just some thoughts but I will be excited to see how you handle it and how it comes out.
SA