Newbie hoping for help with Kohler trim replacement

I am looking to replace my Kohler made brass trim (shower only with two handles) with silver-toned trim. The tile is in great condition and I do not have any extra, so I don't want to replace the valve body. When I get down to the cartridge, it is an old style (~12 years) which Kohler no longer makes. I have tried their customer service to see if there is a conversion kit, but they weren't sure what I was talking about and couldn't help.

The end of the cartridge is square on three sides and rounded on one side - it kind of looks like an upside down slice of bread. Does anyone know of another brand that may have trim kits that would fit this style of cartridge or a conversion kit to a more universal style? I'm attaching pictures showing the different pieces in hopes that seeing the issue will help. Thanks in advance!