I'm quite surprised that the Willco boards are even still around, much less available. I should note that I know of at least 2 that have told me that installation of those boards is not for the average hacker (one is an extra class ham, and works in the broadcast media center as a technician). If you do go this route, line up someone that will do it for you.
Another source of replacements for that battery is here...
ICOMPROM ICOM Non-Volatile RAM Module - $69.00 : Piexx Company, Computers & Electronics
I know of more than a couple of R71s that have had the PROM fail - not surprising, given the age of the radio. As I understand it, the battery only comes into play when the set has no power. If it's left connected to the AC all the time (not a recommended practice during lightning storms) you probably won't have an issue.
Better safe than sorry...Mike