THE FLT DEPARTED OKLAHOMA CITY, OK WITH EN ROUTE STOPS PLANNED AT AMARILLO, TX TUCUMCARI, NM. AT TUCUMCARI, THE OCCUPANTS FOUND THEMSELVES 'STAGGERING A LITTLE' & CONCLUDED THIS WAS FROM EXPOSURE TO HI ALT (EN ROUTE 6500 FT, TUCUMCARI, 4065 FT). AFTER LEVELING AT 9600 FT ON THE NEXT LEG OF THE PLT, THE RGT FNT SEAT OCCUPANT BECAME NAUSEOUS, VOMITED & FELL ASLEEP. THE PLT BEGAN FEELING SLEEPY & PASSED OUT (FROM CARBON MONOXIDE). THE ACFT BEGAN A CIRCLING DESCENT & EFFORTS BY THE BACK SEAT OCCUPANTS TO REVIVE THOSE IN THE FNT SEATS WERE UNSUCCESSFUL, EXCEPT BRIEFLY, WHEN THE PLT ASKED 'WHERE IS THE RWY.' A 15 YR OLD PASSENGER IN THE BACK SEAT TOOK CONTROL OF THE ACFT BY REACHING BETWEEN THE SEATS, BUT THE ACFT HIT A FENCE DURING AN EMERGENCY FLARE-TOUCHDOWN. MULTIPLE EXHAUST CRACKS/LEAKS WERE FOUND IN THEHEATER-MUFFLER. THE ACFT HAD RECENT ANNUAL & 100 HR INSPECTIONS. THE MECHANICS REPORTED, EXHAUST AD/INSPECTIONS WERE MADE. THE OWNER HAD PERFORMED SOME OF HIS OWN MAINTENANCE, BUT HAD ALLOWED OPERATION WITH SEVERAL DISCREPANCIES.