I wouldn't fill it, as you'll then have to repaint the section where it was filled, and chances are it will not exactly match the lower portion, which contains the '773" number. If you painted the entire engineer cab section, you'd need to re-stamp the number.
I certainly would leave the '773' rubber stamping intact, and so, I vote for 'leave it as is'.
The 773 is a valuable engine. Any modification will adversely impact its value, particularly if it is in excellent or better condition..
Len Carparelli
L & L Model Train Restoration Co.