Not difficult at all.. if all you're wanting is a single piece seat. Touratech sell them.... just remove the old dual seat and put on the single piece. job done.
If you want to ape the ADV seat.. have the TT seat recovered.

I'm sure someome like Tony Archer can source the material to do a good enough copy.
gets more complicated if you want to put an original ADV seat on an 1150. there are various other parts needed... and tracking them down is the difficult bit. as is finding an ADV seat for sale... they don't come up very often. And tend to get snapped up when they do.