physically replacing it is easy. There are two brackets holding it in place, each bracket has two tiny screws. Remove the screws, remove the brackets, remove the glass. Reassemble in reverse order. You might start with just cleaning. Mine was filthy when I got it. It looked like someone had spilled coffee on the inside of the screen. So I removed it and carefully washed it in warm soapy water with my fingers (no rags or abrasives), and that made it satisfactory for me. You can look for replacements, but unless the seller knows the exact size of the Rittreck screen you'll need to measure your screen to compare.
The rule of thumb that I've heard is courser grinds on the ground glass make it brighter, but make focus harder/less accurate. There are companies that do "bright screens" like Maxwell--but I don't know what they do to make it brighter, and they tend to be expensive. If I was going to replace mine, I'd probably go with this:
https://www.ebay.com/itm/275219798875
Its cheap, hand ground, and on schott glass, and from a USA seller that I've heard good things about, but I've never used him, and its not a "bright screen". He claims that hand ground is brighter than acid etched, but I can't confirm that.
Also his screens don't come with markings (some may prefer a grid or similar), but he can notch the corners for free.