I wouldn't be too paranoid about using the exact coil. A 23-800 or 23-900 would also be ok. The 23 part is the wire gauge and the other number 850 in your case is the number of turns. The 23-800 will have slightly more "punch" If you get a 23-900 and wanted to you could unwind 50 turns, but I wouldn't bother - particularly for a drop target reset.
WMS used anything from 23-750 to 23-900 on various sys3-6 machines I've got, all with the same driver board and to be honest I can't pick any difference in game play. I try to make sure that slings and bumper groups use the same coils, but don't really think it matters that much.
Make sure you have the diode on it for DC use, correctly orientated - banded end to positive, and find out why the original coil failed in the first place so you don't stuff another one.
