Brian Whatcott wrote:

A hole in the motor case, for a tube and small bronze bellows,

Don't even need that complication. The existing seal should be adequate
for a compensated system. Drill and tap the casing for a tube fitting
and attach a foot long piece of surgical tubing. Fill the motor with
light biodegradable hydraulic oil, and top off through the tubing. Stick
a rubber plug in the tubing end to seal it and it's done.

Rick