if its the same as the clutch lifter rod on the 500K0, K1, it gets pushed through the crank case horizontally to compress and release the clutch springs by a worm gear mechanism connected to your clutch cable, allowing/halting slippage of the clutch plates and friction plates when you shift. they're part of the reason from what experts here have said that the 550 has a whole different clutch actuating set up than the 500's. The small amount of clearance between a turning drive chain was often closed by a maladjusted loose chain, causing the spinning chain to basically either bend or break the rod. they come up on ebay from time to time, but be sure to get a pic and ask for a measurement; the one in my bike originally was actually broken in two...Someone here should have the specs on the 350 rod, which can be remade from a steel rod of the right diameter as well.
If your lifter rod is bent, be sure to check the actual mounting point for the clutch cable inside the crankcase cover, as they are often casualties of a whirrring chain as well. The steel worm gear is seated in an aluminum housing which has very thin walls. Check to see if the walls are cracked too, as this issue will limit your ability to adjust the clutch using the wide slot on the outside of the crankcase cover...cmsl has some of the parts for the 500; bryanj or Hondaman can probably tell you if they are interchangeable with your engine.
good luck,
hym
