There are several types of replacement vent covers that are available, most at nominal prices.

The powered vents take a little more time to change the covers, but basically are the same as the non powered. Most of the work is done on the interior to gain access to the actuating lift rod. Once that is removed, you can go onto the roof and slide the old cover off. The hinge area is what secures the cover to the frame and roof. A new cover is slid into place, no tools needed. Then it's back inside and the garnish, screen and rod are replaced. Sometimes the motors are mounted onto the screen frame.

About the only tool that is needed is a #2 phillips screwdriver. The hardest part is dragging a ladder about to get onto the roof.

There are many qualities of replacement covers, with varying life spans, and resistance to UV damage. If your time and labours are worth anything, the more expensiver ones last much longer and are available in different materials.