I have operated over that track with my speeder on Motorcar Operator's West excursions and later a few work parties to beat back the vegetation. Last time about a decade ago.
Based on my observations NONE of the track, with the possible exception right around Cloverdale (Caltrans put in new track when US 101 was relocated around Cloverdale), is in "good shape" for passenger operations.
But just to speculate: To Cloverdale is always possible, just requires entirely new track. But no nasty slides or tunnels.
Ukiah is unlikely due to the high cost for the extensive work needed in the Russian River canyon section between Cloverdale and Hopland to the tunnels and slide area's. Last work party the track was barely passable due to slides.

Don't get me wrong, I would love to see rail service restored!