As seen here: https://ogrforum.ogaugerr.com/t...16#72805549161148316
Got a 244 in the mail from a Mercedes-Benz chum of mine, and I decided to recreate my very first train set from 50 years ago...
It was a mish-mash of mixed pieces that my father had gotten from a work chum who had an extensive Lionel collection in his basement, but I loved it to death and it still exists in the care of my nephews in Boise.
This 244 was missing a marker, so I decided to replace it.
Step 1: 5/16" styrene tube, cut to length.
Step 2: Cemented in place with Testor's styrene cement.
Step 3: After the cement dried, a little Testor's model putty in the back and a dab of flat black paint.
Step 4: Added jewels for that Lionel bling look. 
You can hardly tell the repair from the original! 
This method is usable for most locos with a missing marker; if the boiler is diecast, I recommend using JB Weld instead of styrene cement.,
Mitch
Pete