"If yes (or even if no), what is the easiest way to make another
stringer to match the current two?

==============
Now, kids, he DID say EASIEST, not "Most Proper", right? OK then.

Easiest: trace a template from the edge of the existing stairs, being
sure to include the edges of treads on the drawing. Cut it out like
paper dollies and lay it on the new stringer stock. Now just gnaw it
out, Bucky.