DURING A WX BRIEFING, THE NON-INSTRUMENT PLT WAS ADZD THAT VFR FLT WAS NOT RECOMMENDED DUE TO LOW CLOUDS & ADVERSE WX ALONG THE ROUTE. HOWEVER, HE FILED A VFR FLT PLAN & DEPD ARCATA, CA AT 1240 PDT. AT 1339, IN THE VCNTY OF WILDERVILLE, OR, THE PLT GOT A WX UPDATE. HE WAS ADZD OF ISOLATED TSTMS & BROKEN TO SCATTERED CLOUDS TO 4000' WITH AN UPPER LAYER AT OR ABV 8000' & SCATTERED RAIN SHOWERS ALONG THE COAST WITH 1000' TO 2000' SCATTERED CLOUDS. MOUNTAIN TOPS WERE FORE- CAST TO BE OBSCURED. THE PLT'S LAST TRANSMISSION WAS AT 1356 PDT NEAR SEXTON MTN PASS, WHEN HE ASKED FOR THE MEDFORD ALTIMETER SETTING. THE ACFT DID NOT ARRIVE AT THE DESTN & WAS NOT FND DURING A SEARCH. IT WAS FND BY HIKERS ON 4/27/90, WHERE IT HAD COLLIDED WITH TREES ON STEEP MTN TERRAIN AT AN ELEV OF APRX 1800'. THE ACFT WAS BADLY DMGD BY IMPACT & FIRE. NO PREIMPACT MECH PROBLEM WAS EVIDENT. DURING IMPACT, THE PROP SEPD FROM THE CRANKSHAFT & BOTH BLADES HAD 'S TURN' DEFORMATION, LEADING EDGE DAMAGE & CORDWISE STRIATIONS.