This is a
screen grab of the frames, 77 vs 80.
You can see that they mount differently. And are probably differently sized, no way to tell from the picture.
But if you can purchase the 80 skid plate, it might be modified with some grinding to fit the frame, and using
this style clamp, could be attached to the frame. Or you can use similar clamps to attach your own hand built skid plate to the frame.
This would all depend on the cost of the 80 skid plate, if you could find it inexpensively, it might be worth a try, if it was costly i wouldn't try it!
In older times I've heard they used coal shovels fastened to the bottom of the bikes as skid plates, probably when Harley Davidsons were the only dirt bikes...
In fact, when Presto Petty made plastic universal skid plates, they were very much shaped like a shovel when I think of it.
If you know someone in your area who works with carbon fiber and could build you a custom skid plate, that would be cool ;-)
Now I'm thinking outside the box, although I did know someone like that!
Post your progress whatever you do.
Good luck with your search.