Dan,  If this is like the Mira, and it probably is, the spring
assembly has a wrap-spring clutch.  It is a rather stiff spring made
of _flat_ steel stock that is wrapped several times around the winding
shaft.  The end of the spring is screwed to the bedplate nearby.

When the crank is turned in the 'wind' direction, it loosened the grip
of the clutch and the shaft will turn easily.  When the shaft tries to
turn the other way, the spring tightens on the shaft, preventing it
from turning.

Now, making a replacement could be sort of a problem.  I suspect that
finding an identical one to buy outright would be next to impossible.
Since it doesn't deform much in operation, I suspect that you might be
able to make one from round steel wire (something like 0.100" Diam.).

You probably don't even need spring steel since the deformation will
never approach the elastic limit of the material.  Just wind the
spring on a mandrel that is a bit smaller than the shaft, to allow for
spring-back after you wind it and for a snug* fit on the shaft.  Then
bend the loose end to fit over the screw.  There should be a threaded
hole for this screw near where the shaft passes through the casting.
Use a steel screw, not brass, for strength.

Regards
Craig Smith

* Please excuse the technical terminology.

[Later] Addendum -- Based on the pictures you sent me, I can see that
the design of your Stella box does _not_ incorporate a wrap spring
clutch like the Mira.  However, I have forwarded the pictures of the
Mira clutch to Robbie so he can add them to the archives in case
someone else can use the information.

 [ Thanks, Craig, I'll place the photos at the MMD Pictures site,
 [ http://www.mmdigest.com/Gallery/Pictures/  -- Robbie