The 6v battery died on my 51 last fall while at a week long chuck wagon race event (
http://www.chuckwagonraces.com/). Called the local O’Reillys and they had one in stock. Exactly the correct size with a resent production date. These batteries also fit a lot of old farm tractors which is why I suspect it was in the store.
If your store doesn't have one is stock they should be able to order it. As I recall it was a little less than a $100 maybe high 80's.
added in edit: Try
thisGood Luck
Ron