A year ago I thought a Singer 66 treadle machine was a demonically inhabited piece of black magic and you needed a exorcist on retainer before you could look inside one. What I found out is that "most" of the old machines are incredibly simple.
The 50s and 60s ZZ machines are simple too, but not like the old SS machines. You've just got to take the covers and plates off and play with the controls as you watch what they do. Eventually you'll get the idea of what pieces do what when you move which control. Oh yeah, each thingy will usually have about three functions.
The ones that really get complicated are the ones with built in cams. In those each thingy usually has four functions.

So you just gotta grit your teeth and jump right in.

We've got over 30 .... or 40 machines and I have only one I haven't been able to diagnose yet. Well, two, but one is being a cranky old Kenmore and the other one has been messed with by someone with a cabinet screwdriver and a case of stupids. I'll get 'em figgered out eventually.