I've seen fairings off Aspencades with all the accessories, and off Interstates, and I can tell you that the Aspencade fairing will have lots more holes in the inside upper fairing than the Interstate will have. So if you buy an Aspencade fairing and use it on a stripped Interstate, you'll have lots of extra holes to deal with. The extra holes are for speakers, gauge pod, etc, etc...
'81 had a Candy Muse Red, so beware that the seller may call it Wineberry, but it's not. Best to know exactly which year the donor bike was. The color of the stripes is a big clue too.
I'd like to find a clean front fairing. I've found some damage on mine that was haphazardly fixed. It works structurally so I shouldn't worry about it but it irks me. May try a better fix myself if possible. I seriously shouldn't even worry about it but it's going to be a long while until the front fairing goes back on so im looking around.
A good move is to check the fairing support frame for straightness, you can damage a good fairing just by bolting it down to a bent frame, check out the damage on your fairing and decide if you think it came about by impact, speaking thru pure experience!