The ornamental design for a powerunit, as shown and described.
FIG. 1 is a perspective view of a power unit showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a back elevation view thereof;
FIG. 4 is a top view thereof; and,
FIG. 5 is a left side elevation view thereof.
The even-broken line shown in the drawing views is for the purpose of illustrating unclaimed environment only and forms no part of the claimed design. The dash-dot line shown in the drawing views is for the purpose of indicating the bounds of the claimed design and forms no part of the claimed design.