I have not used the Krylon Fusion on any saw parts, but I did use some of the black to refinish my outside mirror on my 2001 Tundra. The clear coat started peeling so I stripped both down to the bare plastic and painted with the Fusion black. Paint went on easily and has held up to bug guts for about four months now.
The car industry has this solved. Most bumpers are now "plastic" (there are MANY types of "plastic") and the finish is excellent. It's definitly all about prep, and the correct primers etc applied at the correct temperature/humidities.
Before painting you need to identify the exact plastic to get the correct primers etc. Most have a tiny molded code (on the back side) indicating the type. Example - on a Stihl you'll often see PA66-GF. This basically means "nylon", glass fiber field. There are other ways to identify "unknown" or unlabeled types.
Here's a good data source on plastics, but probably much more then you'd ever need! http://ides.com/
I used the fusion paint on my sons dirt bike, #71 below. I holds up good but is not fuel resilient, even with good prep and a week to dry it peeled where the fuel leaked on it around the gas cap.