The spool can is the biggest concern, a very badly worn one may not work properly. Also the thread tension assembly is occasionally missing or worn out, so check for that.
The spool plate is sometimes missing (unless it's the attached kind that swings out) but it's fairly easy to make a reproduction if you're handy with simple metal working. For some reason mine was missing her spool pin up top, and modern press-in spool pins are much smaller and don't fit, but again reproductions are fairly easy to make.
Wooden spools that fit the spool can are easy to find. Just make sure they aren't bigger than 1" across and 1 1/8" tall with a hole at least 1/4" wide, or they might not fit.
Two Spools, at least the one I have, where designed in such a way that it's difficult to remove most of the mechanics without having to re-set the timing or breaking them. Whether this was on purpose to get people to take them into shops for repairs or just incidental to the design I don't know. It does mean that most Two Spools rarely have anything missing but the spool can. Good for us!