I checked out my modern day options too, on the Windows side and it seems there's a really nifty web solution for this. BatchPhoto Espresso. PICT is right there in the first choices in the pulldown menu!
my steps were:
1) in Gimp, change the image to indexed color, 1-bit black and white. I'm sure you can do this in photoshop as well
2) export to BMP
3) go to
https://www.batchphoto.com/espresso/
EDIT - this route does NOT work. The website exports to a blank page, it does not recognize the 1-bit bmp in the right hand side view, even though the preview shows up on the left side, at the upload step.
EDIT - route 2:
export to png instead from gimp
use batchphoto espresso to pict
import to System 7, Aldus Superpaint 3.5
Opens fine
Using that, I can save to an older MacPaint format, which opens fine in Claris MacPaint 2 under System 6. I assume I'd be able to open it with the original MacPaint as well