Hello,  Having dealt with several vintage piano motors over the years,
I can offer this method which has served me well.

Reinsert the bolts, but instead of passing them through the holes
in the bell housings, allow them to touch the inside of the casting.
A long, narrow screwdriver will work too.

Obtain a rubber mallet or block of wood and _gently_ tap each bolt
in a criss-cross manner, i.e., at 11 o'clock, 5 o'clock, 1 o'clock,
7 o'clock.  Tap each one _gently_ and repeatedly while holding on to
it so it doesn't bounce, and the end bell will eventually pop off
nicely.  You'll see a gap start between the motor housing and bell
casting when you're making progress.

Repeat the procedure for the other end, but look through the space
between the case and field windings where you will see air gaps used
to facilitate cooling.  Tapping through the gaps will remove the other
bell housing, if needed.

It's very important to mark the case and bell housings so as to make
realignment for reassembly easier.  There is no keyway, per se, on the
motor case to facilitate alignment.  Make sure that the thrust washers
present remain on the shaft and don't stick to the bearing where they
can fall off during reassembly.

Did I mention to do everything _gently??_

Best!
Bob Hodge