It is very tempting to perform quick, cheap, and easy repairs on
parts of a player piano in the vain hope that it will then perform
beautifully for another 80 years.  It won't!

If the existing cloth on the wind motor is leaking at the folding
corners then there is every chance that it is completely worn out,
and will perform poorly regardless of any patching.  Rip it all off!
And while you are at it, check the wind motor governor!

The choice of replacement cloth is obviously important, and a
moderately light bellows cloth seems the correct one.  Whether or not
cotton drill 40 cloth meets the requirements I am unable to say.

The wind motor (or its governor, for that matter) is not the most
difficult item on a player piano to restore, and I would encourage Erik
to have a go himself if cost is an issue.  Careful noting of the span
dimensions and slider positions will ensure proper operation following
the rebuild.  As for the corner punchings, I use tan pouch leather --
but the sheepskin alternative should be easy enough to get from PPCo.

Erik does not say which arrangement of pneumatics the motor comprises.
If it is a set of single span pneumatics then he must be careful when
separating them from the trunk to ensure no damage.  If it is of the
triple double-acting Aeolian type (a thing of beauty indeed), then
there is also a requirement for pouch leather on the areas of overlap.
A small bottle of liquid graphite for application and polishing on to
the trunk will finally ensure smooth working.

One day's work at most either way, Erik!  Good luck!

Roger Waring - The Pianola Workshop
Solihull, West Midlands, UK
http://www.pianola.co.uk/