Complex sentence: He was originally a runner-up in the "R U All That" competition only losing to; but soon joined the cast as a replacement for Jamie Lynn Spears after she left "All That" to film Zoey 101.
Split into simpler sentences: He was originally a runner-up in "R U All That?" only losing to. Ryan eventually replaced Jamie Lynn Spears after she left "All That" to film Zoey 101.