There are a lot of unanswered questions that I have about the problem.

I have seen a problem with the drive chain that could easily cause
the problem.  On the right side of the tracker bar is the transmission;
it has a gear that moves right or left and is connected with a chain.
Check that out as a possibility.  There's a small bobbin that keeps
the tension on the chain (if memory serves correctly!) -- make sure
it's in place.

If you put the play lever in the play position, is it jammed or does
it turn?  If it turns, does it turn freely, as if nothing is attached,
or do you then move the motor (manually)?

If you put it in play mode and pump the pedals with holes over the
tracker bar, do the notes play?  Is there any difference in how the
notes play (than it played before)?  Is it easier or harder to pump
the pedals?

John Tuttle's suggestion about the Tempo governor is a possibility.
Also have someone pump and listen closely for air leaking around all
tubes.  The stack is powered by a direct connection to the left of the
keyboard.  The air-motor governor should be underneath.  You don't need
to check the small tubes behind the stack for the tracker bar; in this
case as it's a major failure.

Eli Shahar