After the fourth touch-and-go landing on RWY 17R, power was applied and a normal climb initiated. At about 400 ft AGL, power was reduced; the engine developed heavy vibrations and approximately 50% power loss. The CFI took control from the private pilot receiving dual instruction and initially turned right followed by a left downwind turn. At 45-60 degrees angle-of-bank, the aircraft turned approximately 190 degrees in an effort to land on RWY 35L. According to the CFI, the aircraft stalled and contacted the ground left wing first. Examination of the engine revealed that continuity in the mixture control system did not exist. No attaching hardware that would connect the mixture linkage to the carburetor was installed by the operator 13.8 flight hours and 35 days prior to the accident. Wind at the time of the forced landing was from 180 degrees at 18 gusting to 28 kts. After re-rigging the mixture control linkage and repairing impact engine damage, the engine was successfully test-run.