If you have 15% leak down loss with a rebuilt head (which usually means new valves properly lapped or old valves properly lapped) I would agree that the rings are about done.
But you should know these engines use oil, so that should not be a concern (unless it uses more oil than gasoline).
If the plugs are fouling, I’d just go up one heat range on them. I’ve done this as my oil consumption is a bit high and my plugs look good upon inspection now. How do the plugs look in your car? You can learn a lot “reading” them. | 
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