If everything is on time, you will probably be fine. But there are a few things working against you. First, August is a heavy-fog time for SFO, which means your inbound could be delayed (though you could only hope your connection is similarly delayed). Second, United has had horrible on-time performance these last few months. Check the UA boards. Third, United has been putting some flights in Terminal 1. If you have an inbound in T1 and outbound in T3 (or vice versa), you would need to catch the shuttle, which may take a while. Lastly, flights to KOA are not that frequent, and subsequent flights could be full. I suppose you could be re-routed via another Hawaiian island, if needed.
All in all, I would try to get yourself a longer connection, if it is not too late.