The "mattress" style seat from any early /7 will fit. The mounting is pretty much the same. I took a "mattress" style seat off a 77 R100/7 and installed the cowl seat from an 82 R100. Fit just fine. So going the other way should be just as easy.
As to a centre stand, it would be much easier for you to try and find one on EBay. But, you will have to remember that the 1981 year was a change over year for centre stands and only ones for 81 will fit. I know, as I tried both 1980 and 1982 stands, and could not get them to fit properly on my 81 R100RT. I eventually found a brand new stock stand on EBay that some guy had taken off when new as he had put on the Reynolds Ride-Off stand. I did the opposite, as I prefer the stock stand.