Your problem sounds more like a bad start cap than the motor, Less than 20 bucks may fix it...
However That Styrofoam block you mentioned, well there is a way to open it up, what that is I can not tell you since mine is different.. Once you get it open and the fan removed from the other end of the motor shaft and the mounts released the thing lifts out, then you can loosen the set screw and remove the blower.
But unless that fan/blower is hard to turn when you first go up on the roof. I'd sooner believe the start cap than the motor.. IF however it is hard to turn, you called it.
What make and model A/C.. (I mean if it's the right one, and I'm sure it is not, I have a spare).