Without having the luxury of drawings, I will attempt to help you here.
I use a woven fishing line to replace lost or broken string; it's very
flexible and tough, and doesn't stretch, _and_ it can usually be found
in sporting goods stores (availability is an important feature!).

So, after removing whatever remains of the old string, tie one end of
the new string to the winding shaft.  Put about four turns of string
on the shaft, then with the "fish pole" at it's farthest distance,
tie the string to it.

Now wind the string so the pole is at it's mid-point; this should have
put the roll in the center.  Do make certain the drum and the winding
"flap" are clean and not sticky; this allows the "flap" piece to release
from the spinning drum when the roll finger tells it to.

Both fingers are connected to the mechanism with rods with adjusting
areas; the wire rod is bent into an "s" shape for the adjustments.
These usually don't go out of adjustment, so replacing the string
should get things right again.

I think it's a very reliable tracking system, and doesn't take any
vacuum to operate -- and there's no cloth to harden nor leak either!
Do lubricate with a very light oil where the shafts go through plate
bearings.

Hope this helps.  I'll bet that John Tuttle's web site, Player-care,
has illustrated instructions, but it takes to long on dial-up Internet
service for me to check; sorry!

David Dewey