Richard and group,  A light dimmer installed in series with the AC line
will work, however, I would use a fan speed control, which has a heavier
triac and better heat sink.

If the rewind is still too fast, perhaps you could swap the gears from
the spool, and driven shaft, changing the gear ratio.

Some rebuilt players are so airtight that they require less suction to
play than rewind.  You may think about installing some electrical lever
switches, that will switch the suction box over on its own speed
control during rewind, and adjusting it separately.

Andy Taylor