Original MX screens are pretty hard to find now.
Pentax LX screens fit the MX perfectly and are a bit easier to find.
They are a tiny bit brighter, but not enough to require recalibration of your exposure meter.
I have a brand new SB-1. It's in the original box and has been opened maybe 2 times for a total of about 20 seconds since I bought it 20+ years ago. I'd love to trade it for a clean Nikkor enlarging lens. ;-) PM me if you're interested.