The wmo file format in 3.3.5 is virtually unchanged since Vanilla. So the WMO editors for 3.3.5 should work in vanilla.
Few tools allow WMO conversion or modification right now. I know off none that work for 1.12.x. Probably the older files within our resources section - most of them are based on vanilla wow. But no idea where else you could find content for 1.12.x. You would have to analyse that yourself.