Broken Balance Cover for Simonton Window
If the only thing that's broken is the cover, then you can probably get away with just replacing that. On the other hand, the fact that the cover broke in the first place could be indicative of an issue with the other parts.
I also recommend removing the entire assembly and giving it a once-over to see if there are other visible signs of damage. You need to remove it anyway before you buy anything, so you might as well use that as an opportunity to check on the other components.
When the hardware is out, tell me the depth of the spring (it should be either 1/2" or 3/8") and I can recommend a cover. I believe yours is 3/8", which would indicate our 15-205 triple cover. But that's not something you want to guess on.
The springs also have stamps printed on the coil band that tells you how much weight each one supports.