I looked at a similar gearbox's instructions and this is what was suggested:

3. Once you have finalized the exact position of the motor in the
gearbox, add a small drop of medium CA to both sides of the
motor where it fits into the gearbox. This will assure that the
motor stays in position.

Like you, though, this doesn't sound ideal, but CA tends to have poor sheer strength so twisting it of the gearbox may not be that difficult, especially if a bit of debonder could work it's way in there.