Scotty Greene inquires about "string driven tracking mechanisms".
These are frequently called "fishing line trackers" with good reason:
that's the best material to replace a broken or missing original with.
However, _avoid_ "monofilament" type line which is typically a plastic
of some formulation and will frequently form a "curl set" if left
inactive for long periods.

I believe my small reel of the stuff, which will last me the rest of my
rebuilding days, is a very light (50-pound test?) nylon multi-filament
line.  Probably the lightest gauge/smallest diameter you can buy will
be adequate.

Let the "arm" travel to its maximum excursion, measure the distance to
the pulley and cut a piece about an inch longer.  Tie the knot at the
pulley in such a way so as not to impede the line winding up smoothly
on it.

No calibration or adjustment (of the string) is necessary.  It will
wind up to the optimal point whereupon the friction clutch will begin
to slip and winding stops.  In my experience, they make some of the
most accurate trackers there are, even single-sided models.  Good luck.

John Grant