The ornamental design for a rear part of a motorcycle, as shown and described.
FIG. 1 is a front, left side perspective view of a rear part of a motorcycle showing our new design, with the broken lines showing other portions of a motorcycle forming no part of the claimed design;
FIG. 2 is a rear, right side perspective view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a front side view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken line showing of other portions of a motorcycle are for the purposes of illustrating the environmental material and form no part of the claimed design.