The clock is not attached to the motorcycle battery, but runs off a button battery. Another post on this forum correctly states that the battery is an LR43 (Maxwell brand). A modern replacement is a Duracell 301/386 cross referenced as SR43.
The clock is accessed after removing the front handlebar cowling and loosening the dash assembly. Be careful with the screws and tabs. So much plastic to break.
Once the rectangle underside of the clock is exposed, unscrew and remove the one screw on the right side of the panel. Pick out the panel, rubber mat and clock mechanism. Removing 4 super tiny screws allows the back of the clock mechanism to come off and the battery is there to pick out.
I'm reviving a 2004 People 150 with 18k on it. These forums are great..