Is it possible to use an older GE service panel as sub panel?
It looks like that panel has two grounding bars.  One on each side, mostly hidden behind insulated wires in your picture.  It also appears like the Neutral bus bar is distinct, and care was taken to keep neutrals and grounds on their separate bars.  This will save you much time.

By the looks of your picture, the panel chassis is used as part of the ground bus, which is kind of sketchy, but ok.  I would also guess the brass screw on the neutral bar doesn't have any hole to put a wire through, and touches an unpainted section of the chassis.  if so, removing it will likely separate the ground and neutral plane in that breaker cabinet.

After doing that (and testing with a multimeter that ground and neutral are separated), you should be able to run this panel as a sub off of a 100A breaker in the new panel.