(ET) replacing Electric Ox power-off brakes

[Ken Olum <kdo cosmos phy tufts edu>]

Hi, all.  I'm trying to repair the brakes on my Electric Ox, so I don't
putt my life at risk, as happened to Chris Zach,
http://cosmos.phy.tufts.edu/mhonarc/elec-trak/msg19652.html

Like many things that ought to be easy, this one isn't.  Maybe
someone will have some thoughts about the best way to proceed.

The Ox has power-off brakes, i.e., spring applied, electromagnetically
retracted brakes, one for each drive wheel.  The brakes sit on the back
end of the shaft of each motor.  There is a disk with friction material
that is clamped between metal disks except when power is applied to the
oppose the spring.  Shortly after you take your foot off the pedal, the
springs are released and the tractor comes to a stop.  Or it would
except that the friction material is mostly worn off.

So I would like replace these brakes, or at least the friction disks.
But I have no idea who made them.  Naturally there's no nameplate.  The
tractor manufacturer is out of business.  I've look at a fair number of
places that make this type of equipment and seen nothing that looks like
what I have.

Alternatively I could buy some different units that would do the same
job, but then there is the problem of attaching them.  In the ones I
have, the friction disc goes over a hex drive collar (I don't know the
right name for this), which attaches to the back of the motor shaft by
means of a roll pin.  The actual motor shaft is much larger, but it
tapers to 3/8" diameter at the end where the hex drive attaches.  I
suspect it has been machined down.  The diameter of the hexagon (between
opposite sides) is about 0.693".

See pictures at
http://cosmos.phy.tufts.edu/~kdo/Ox-brake-1.jpg
http://cosmos.phy.tufts.edu/~kdo/Ox-brake-2.jpg
http://cosmos.phy.tufts.edu/~kdo/Ox-brake-3.jpg

Possibilities seem to include:

Replace the entire unit.  But then I need a power-off brake that fits on
a 3/8" shaft with a roll pin.  Everybody seems to use square keys
instead.  Except for this problem, I think the Inertia Dynamics FSBR007
would work.

Keep the hex drive collar and replace the rest.  Then I would need a
unit that uses a 0.693" hex drive.  Inertia Dynamics uses 5/8" = 0.625.
I couldn't find anyone whose hexagon is the right size, and there's no
way to search for that.

Modify my hex drive collar to make the hexagon smaller so it fits the
Inertia Dynamics or some other unit.  I don't know how to do that, but
perhaps it would be straightforward for a machine shop.  Or buy or make
a different hex drive collar.  I don't see them for sale, perhaps
because I don't know the right name of this part.

Somehow rebuild or repair the friction disks with new friction material.
Can that be done?  Or get someone to fabricate new friction disks for
me.  This has the disadvantage that if something else goes wrong I'll
the back to square one, whereas if I figure out how to mount new units
to my shafts I'll be able to replace them again without trouble.

Any ideas?  Thanks!

                                        Ken