Hello Hans_klokkie,
a couple of years I was faced with the same question.
I found a Spoiler / Belly Pan basically designed for BT1100, manufactured by Ricambi - Weiss. Unfortunely they do not produce it any more.
see first Picture, secound picture shows the reshaped Ricambi-Belly pan. There was a little bit of cuttting-off work neccessary, depending on the position of the forward Exhaust Pipe. Further I wanted to see the Oil-Level-Inspetion-Glas, without removing Parts.

I just had quick searchon the Internet. "BT1100 Belly Pan" brings some results for your further going investigation.

Next Winter or After-Season-Inspection I want to redesign the Mounting of my Belly Pan with adjustable Screws.

Hope that will help you

Regards Anja