My guess is that the trackball design in the M5-1 has an intrinsic flaw. Very little force holds the roller up against the ball, and so the ball doesn't grab the roller reliably (on mine anyway.)
My guess is that any other M5-1 is going to have a similar issue.
Now maybe you could lube the roller's axle, to reduce friction there, or apply some tacky substance to the ball or the surface of the roller, to increase fiction there.
It's possible that my M5-1 is missing a part that would cause it to work better. The board was in great shape, it looks like a low-mileage board, but that doesn't prove it's complete.
The trackball on the M5-1 is tiny. Even if it worked, the size isn't great. Though the location is decent.
Kishy, you probably don't want just the trackball assembly, because you'd need more than that to convert a standard model M to an M5-1. The M5-1 has a different controller, a different barrel plate, and a different top half of the case, plus its unique trackball assembly.
