[Biker Biggles]

You probably have more than one problem with it. The charging issue requires a proper diagnostic check as said above. No point in just buying new battery/rectifier etc without checking everything first including alternator output and wiring connections.
The poor running might be related to all that but is likely to be something else. When was the bike last serviced? That means throttle cable play,choke cables/plungers lubed,plugs cleaned/changed(wet front plug clean out drain hole),carb balance,valve clearances to name a few often ignored items. Its nearly twenty years old and needs a regular critical eye to keep it sweet.