The ornamental design for a side gratedguard of a motor scooter, as shown and described.
1. Side grated guard of a motor scooter
1.-1.6 Motor scooter 1.1 is an elevation view of a side grated guard of a motor scooter, showing my new design; 1.2 is a front perspective view thereof; 1.3 is a front elevation view thereof when installed on an exemplary motor scooter; 1.4 is a top perspective view thereof when installed on an exemplary motor scooter; 1.5 is a side view thereof; and 1.6 is a top view thereof. The broken line showing part of the guard and motor scooter are provided for the purpose of illustrating environmental structure and form no part of the claimed design.