I can't imagine they would be that far off in weight. Happ lists their replacement ball as being made of polycarbonate. Acrylic is a similar material. I know it is possible, but I don't think it would be like the difference, say, between steel and tungsten.
The only weights I saw listed for any of the three sources here said a 2 1/4" ball is 5.6 oz. Knowing some folks have used 8 balls for their trackball, I checked the regulation weight of them. It should be between 5-6 oz, so these balls seem to fall within an acceptable weight range for a replacement trackball. I can't attest to another size, but I think it's safe to say it's about the same.