THE PLT RPRTD THAT WHILE HE WAS CRUISING AT 13,000 FT ON AN IFR FLT, THE ACFT WAS SKIMMING THE TOPS OF RAGGED CUMULUS BUILD-UPS. HE ESTIMATED THE ACFT WAS APRX 200 TO 300 FT BELOW THE TOP OF ONE SUCH CLOUD WHEN SEVERE TURBULENCE WAS ENCOUNTERED. DURING THE ENCOUNTER, A 1500 TO 2000 FT LOSS OF ALTITUDE OCCURRED & THE ACFT WAS SUBSTANTIALLY DAMAGED. AFTER THE OCCURRENCE THE PLT NOTED THE 'GEAR UP' LIGHT WAS NOT ILLUMINATED, THE AIRSPEED INDICATOR WAS INOP, & SOME RIVETS WERE POPPED ON THE WINGS. AT THE DESTINATION, HE WAS UNABLE TO EXTEND THE LANDING GEAR WITH EITHER THE NORMAL OR EMERGENCY SYSTEMS. HE THEN DIVERTED TO REDBIRD ARPT & MADE A WHEELS-UP LANDING WHICH RESULTED IN MINIMAL ADDITIONAL DAMAGE. THE PRE-FLT WX BRIEFING CALLED FOR TURBULENCE ALONG THE ROUTE WITH OCCASIONAL LIGHT TO MODERATE ACTIVITY BELOW 8000 FT. THE PLT RPRTD HE EXPERIENCED SMOOTH CONDITIONS PRIOR TO THE ACDNT. AFTER THE ACFT WAS PLACED ON JACKS & THE LANDING GEAR MOTOR RELEASE HANDLE WAS 'JIGGLED,' THE NORMAL & EMERGENCY GEAR EXTENSION SYSTEMS OPERATED NORMALLY.