Here's some thoughts...
1) If the opened doors will not show....i.e., as if they were slid entirely behind the adjacent walls...then simply cut them out just inside the side moldings and just below the transom. I'd do this by drilling a hole in the upper corners of the door to accommodate a narrow saw blade for the horizontal cut. Clean it up with a file when done.
2) If you wanted the doors partially opened...or somehow yet visible in the open position...I'd remake them simply from scribed styrene sheet and thin (0.020") strip stock that matches the original doors. Evergreen makes all the materials you need to do this. Then, cut out the original doors as in 1), and glue the new doors into the position(s) you fancy. BTW, were it my own building, I'd try making the doors out of scrap scribed and strip wood. Sealed and painted, they'd be good enough to the casual eye. Better yet....cardstock!
3) Buy a spare Ameritowne wall piece or two that has the same doors. Cut away everything else to retrieve the doors. Cut out the doors of the original building as in 1). This idea, of course, is very expensive and wasteful compared with #2, but it is an alternative.
4) Check with suppliers of O scale doors....Grandt, Tichy, et al. Perhaps they have something close enough in appearance/size to replace the removed doors.
When done, take some pictures and show us the results...and tell us what you did!
Sounds like a fun project!
FWIW.....always.
KD (aka, Lucas Gudinov)