I claim the ornamental design for a seat attachment for a motorized vehicle, as shown and described.
FIG. 1 is a first upper perspective view of a seat attachment for a motorized vehicle, showing my new design;
FIG. 2 is a second upper perspective view thereof;
FIG. 3 is a third upper perspective view thereof;
FIG. 4 is a fourth upper perspective view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines in the figures are for the purpose of illustrating portions of the seat attachment for a motorized vehicle, which form no part of the claimed design.