It is hard to predict what exactly would change if most of the Grand Canyon was designated wilderness.
It is hard to predict because the exact wilderness regulations differ based on many things related to the history, local politics, national politics, economic activity, etc. of the protected area. It is possible something like motorized rafts would be prevented, but not guaranteed. For example, the Boundary Waters Area Canoe Area Wilderness has exemptions on some large lakes allowing motor boats to be used.
Generally, wilderness areas are more restrictive about the types of activities that are allowed, compared to National Parks or National Forests. Typically restrictions are based on technology, and the technologies that are allowed are those that were historically used in the area prior to passage of the Wilderness Act. But precisely which activities those are is a political question that affects the specific regulations that would be written into the designation of the Grand Canyon as a wilderness area.
These things are reflected in wilderness area signage. Some signs list activities prevented in all wilderness areas, resulting in mismatches between signs and locations. For example, a sign at a Boundary Waters Canoe Area Wilderness entry point says hang gliders are not allowed. But why anyone would take a hang glider into the BWCA is a mystery.