Darrell to further assist in your research, the 121st was absorbed by the 16th Reserve Bn which in turn was absorbed by the 1st Reserve Bn in Feb of 1918. (Sister bns of the 16th were the 103rd, 121st, and 225th.) When absorbed by the 1st Reserve Bn , the sister bns became the 30th, 143rd, 158th, and 238th Bns. All of these were western Canadian units. Of most importance to your request is the fact that the 1st Reserve Bn sent reinforcements to the 7th, 29th and 72nd Bns CEF. In youre search for casualties to the 121st, follow Ron B‘s advice and check the casualty rolls for these units, cross-referencing with the service number of the 121st.
Another tack to follow is to find the mobilization area and check the memorials and cenotaphs in the local recruiting area for original members of the 121st.