First off, charge the battery fully, preferably with a smart, trickle charger (see below). Install the new rotor, then test the system: put a voltmeter on the battery, in the bike, run it up to 3,000 RPM, and if the voltage is 13.5+, the system is working normally.
After that, I would recommend two things: a smart trickle charger (very good, even if you have a Gel Cell, or an AGM battery type; especially good for flooded batteries!), such as a
Battery Tender Jr. to use every time you get off of the bike, in the garage -- keeps the battery topped up, without damaging it, extends the life of all battery types; secondly, a voltage monitor to keep an eye on the whole electrical system's health, at a glance, at all times.
There are several types of battery voltage monitors, but the best, IMO, is the
Argus Battery Bug Monitor. This may be the Cadillac model, but it is LCD, so it drains almost nothing, it can be left on, un-switched, for many months before it would drain the battery. It also has audible alarms. I highly recommend this to keep an eye on the electrical system's health, overall. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII