I can’t see how the brush and spring became separated unless the brush popped out while the armature was being removed. Or do you mean that the spring and brush were both in the channel and not in contact. Double check that the brush slides freely in it holder.
The objective of using the silicon is to reduce the brush wire vibration. Use some in the pocket where the brush lead comes into the starter and at the rear of its brush holder. For the second brush, apply the silicon between the half moon post and the rear of the brush holder. Don’t use any the brush and do not inhibit the brush from contacting the armature. I used the clear auto/marine 100% silicon, clear, from a local Auto Zone.
LAD Specialties / tony kart / rotax / kt100