If the plate is cracked above the flange, it is of no big concern,
considering that three-quarter plate pianos had no iron around the
tuning pins.  Also, Mills pianos have far more uniform tension than
conventional pianos.  The flange bears most of the tension.

If the crack is above the flange, I would at least fix it cosmetically.
But if it extends into or below the flange, the crack presents a big
problem.  Plates can be welded, but only by a skilled welder, with iron
rod.  I would seek out a skilled craftsman for this job, because there
will be no second chance.

I would also check to see why that plate cracked.   It is either due to
improper casting, to the lag bolts giving way, or to the back being
racked (twisted out of square, due to bad glue joints).  If the back is
racked, it will have to be broken down to its timbers, and re-glued
square.  This opens up a Pandora's box, including bridge and extensive
soundboard work or replacement.  But considering the rarity of these
machines, it's well worth the effort.

Bear in mind that, if you are lucky enough to find a replacement plate,
the piano will need a new pinblock.  I ran into this on an old Foster
piano: even though the plate fit the back perfectly, the tuning-pin
holes were off in places.

I believe you should try to purchase the machine for less, as it seems
to have a major problem.

Andy Taylor