My wife's client gave us an 84 Honda Spree with 3000kms

What type or 'code' of spark plug do I need to look for when I go to buy one tomorrow.
Same question for the battery - what do I need to get?
Mechanically everything seems to work on the bike except for the rear break line. It seems like the cable has seized inside (front operates fine). Is there anything I can do to get the rear brake cabel 'unstuck'???? I've unhooked the cable from the rear and tired just gently bending it a bit to see if it would unstick but no luck. Should I try a squirt of WD-40 or something?
Thanks so much for any help information......
This is going to be a fun project for the summer I think!
Steve