Apparently, The joe editor has a jstar mode, so says this old Stackexchange thread. I can't verify because I've never used Wordstar, but joe's available in my distro's repository.
No idea if it can read old WordStar files, but maybe you don't need that.
For the GUI version - and some old file capability, the same page and other searches turn up WordTsar which is in progress. The dev says they'll be picking up development again next month.
There’s also WordGrinder, which is meant to do the job of no bullshit, no distractions word processing. It’s a slight step above a text editor in that it can do bold and such but it’s meant to get the words out of your head and onto the screen and that’s about it.
The program’s path from a CP/M app by MicroPro onward is winding, being shoved into a half-baked office suite, acquired by SoftKey, which became the Learning Company, acquired by Mattel, spun off to Houghton Mifflin Riverdeep, and is now the archival property of—well, nobody’s quite sure.
Looking forward to the eventual frivolous takedown notice and/or lawsuit – suits seem to have absolutely zero brains when it comes to this stuff. Or, well, when it comes to anything except making themselves and their buddies on the board richer, really