The basic matchlock appears to be two moving parts (three with the pan cover) and one spring, though it seems it would be possible to get it to fire even if the spring broke. See
http://www.vikingsword.com/vb/showthread.php?t=7524
Simpler than that and we have the basic serpentine:
http://www.vikingsword.com/vb/showthread.php?t=7077
Any simpler than that and I guess we're down to a simple touch-hole. Though in a 19th century setting I guess a percussion nipple&cap and a blunt object could work as well, depending on how much force you get blowing back through the nipple when the main charge goes off. Might make it hard to aim though.