really great ideas..!
Pull the cover off the end thats sparking and you will see there are plenty of little parts to lose in the dirt near your feet. All of them are important and necessary for the thing to work right.
I just thought I might throw in, it will|is working other than the spark right? So sometimes its a better idea to just use it carefully until it fails, or you have the time to really fix it.
A motor doesn't usually spark out unless there is a power issue like a loose wire on a brush or an insulation problem on one of the coils. I have an old drill that arcs to beat the band when you fire it up, but that is because I haven't polished the commutator, and cleaned then insulated the wiring. That looks like you might have a natural buildup of electricity in your wood shed and the static charge is happening when you create a tornado by spinning the blade on the motor, the resulting hurricane causes a small lightning bolt to discharge when the switch is thrown. Put the saw on a rubber mat on the floor, and hand a couple of those vinyl door strips across the dry storage area between the saw and you and see if it doesn't stop.
2. a bad switch can cause a power problem that causes arcing also. Imagine a power switch that turns on and off 25 times before it actually turns off....
3. never put your fingers near the saw blade... or the electrical connections or motor...
4. Always wear goggles
5. Gloves are not always a good idea
6. Spray the motor and commutator with Contact cleaner, then let it dry.