As it is knackered just break it off, put the new one on with a little grease/coppaslip between arm and shaft, they can get extremely stuck on.
When you put new arm on, just check for "ford wiggle", that is play in the slot so the arm moves/rotates slightly on the shaft, if it does, just put a bit of paper over the lip of the shaft and press arm on this will take out any slack.
The cap when you say oxidised if you are just referring to the terminals inside the cap they do this anyway, ie look burnt and pitted, not a reason to change.
tabetha