Paul,
Arceurotrade supply some bellows which may be suitable. However you may be able to salvage what you have. Don’t let it dry out any further, a bit like the Mary Rose. You need a good leather saddle soap or a leather sofa treatment and slowly feed it. Once it supple you can remove section that beyond hope, patch the holes and replace pieces. Either glue or stitch or staple. Even just a gorilla tape mend. Option B is to replace with rubber sheet skirts, but seems a shame to change them if you can restore. Someone with an small industrial sewing machine could be useful and a trawl around charity shops for old leather jackets. I been there done that, have a machine, just the task indoors on a winters night?
Peter