It's a while since I did a Celestina, but yes, the proper way is to
remove the tapered drive pin.  It may be rusted slightly which will
make it difficult to remove.

If you haven't already done so, remove all of the lid, including the
fixed section at the top for greater access.  Put a few drops of
penetrating oil on the pin and leave it overnight (place some paper
tissue underneath to catch any drips).  Then use a suitable drift and
a not-too-big hammer to tap out the pin.  A brass drift would be best,
and a hole drilled a millimetre or two into the end to fit snugly over
the drive pin will help stop the drive pin bending.  Once you have
broken the rust it should come out easily.

If this still doesn't work, then you may have to break or cut off both
ends of the pin and file them down to the shaft so you can dismantle,
then knock out or drill out the stub and make a new pin from some mild
steel rod.  You can usually taper it enough with some emery paper with
the pin in a lathe or drill.  It is normally quite a shallow taper and
the pin goes through more than half way, and it is the "thin" half of
the pin that does the driving, with the thick half short enough to miss
the driven peg and slot.  On re-assembly, just squeeze the pin back in
with long pliers, don't tap it in.  It will not normally fall out in
use and will be easier to remove next time.

In case you haven't realised, it is the screws from underneath around
the reservoir that release the complete mechanism.  The screws accessed
through the top only attach the top panel to the valve chest.  If you
undo the screws underneath (six, I think) the whole assembly lifts out
through the top.

If you need rubber cloth for re-covering the exhausters or bellows,
it is the same as that used on player pianos, and I sell it to UK and
Europe.  See www.musicanic.com

Good luck,
John Farmer
UK