My compressor still works great, but the clutch is wearing out. Makes a lot of dust, and occasionally doesn't want to engage. Is it difficult to just replace the clutch unit?? Does it require any special tools to remove, such as a "puller." I am thinking about finding a replacement at a pick-a-part yard.  Thanks!!

Not difficult. Basic hand tools and snap ring pliers. They don't require pullers but they frequently can be difficult to get off due to corrosion and clutch material. 

Thanks, advise much appreciated.........Rooster2