There are two ways to deal with a broken clock mainspring.  I know
this is a musical box movement, but they're functionally identical.

One way is to replace it, which involves measuring its thickness.
Generally they're on the order of several thousandths of an inch
thick, so you'll need a micrometer or digital calipers.  Measure
the width of the mainspring, which you can measure easily enough,
and you'll need a rough estimate of the mainspring's length.  You
can never unroll them completely, so measure what you can and add
something like six inches or maybe a foot; lengths are not very
critical.  Then have a look at www.timesavers.com and start digging
through their on-line catalog.  Or -- and this might be easier --
send me the dimensions and I'll look it up for you.

Or, you can fix the thing.  Unroll as much of the broken inner coil
as you can so you can get at about an inch of spring.  You may have
to resort to brutal tactics, and a smallish Vise Grip pliers is handy.

Use a small torch to heat up an inch of spring to a red heat and then
let it cool down on its own.  (Do _not_ quench it in water, and you
won't be re-hardening it.)  This will render the spring soft enough to
drill a new hole in (I have a punch kit, but a drill works okay) and
also soft enough to wrap securely around the winding arbor such that it
won't come off its hook.

Wind the spring back onto the arbor tightly to re-form the inner
coil around the arbor and stuff the thing back into its barrel as best
you can.  It'll be somewhat distorted, but this won't affect its
performance.  Nor is it any more likely to break than a new spring.

The shape of the hole is irrelevant provided it holds onto the hook on
the arbor.

Mark Kinsler
kinsler33@gmail.com.geentroep

 [ Caution should be used with any spring disassembley, repair, or
 [ reassembly as springs can store a lot of energy and they have sharp
 [ edges. Losing control of a spring can be very dangerous.  I'm not
 [ brave enough to take one apart myself.  --Jody