THE NON-INST RATED PLT, WHO WAS AN A&P MECH & AN AUTHORIZED INSPECTOR, HAD JUST PURCHASED AN ACFT. ON 12/18/90, HE & ANOTHER MECH TRAVELED TO ROCKFORD, IL TO TAKE POSSESSION OF THE ACFT. THEY WORKED ON THE ACFT & THE PLT CERTIFIED AN ANNUAL INSPN. ON 12/19/90, THE PLT OBTAINED A WX BRIEFING FOR A FLT TO KINDRED, ND. THE WX PREVENTED FLT TO KINDRED, BUT THE PLT ELECTED TO FLY AS FAR AS WX WOULD PERMIT. HE TOOK OFF AT ABOUT 1415 CST. A RESIDENT NR BOSCOBEL HEARD AN ACFT FLYING LOW IN HIS VCNTY AT ABOUT 1530 TO 1600 CST. LATER, THE RESIDENT LEARNED THAT AN ACFT WAS MISSING. HE SEARCHED THE AREA & FOUND THE WRECKAGE WHERE THE ACFT HAD HIT TREES WHILE DSCNDG ON A SOUTHERLY HEADING. THE RESIDENT RECALLED THAT THE VISIBILITY WAS ABOUT 200 YDS WHEN HE HEARD THE ACFT BEFORE IT CRASHED. ABOUT 25 MI EAST AT LONE ROCK, WI, THE 1459 CST WX WAS IN PART: 700' OVC, 1-1/2 MI VIS WITH FOG. NO PREIMPACT PART FAILURE/MALFUNCTION OF THE ACFT WAS FOUND.