The airplane required considerable left rudder in most flight attitudes, so the pilot fabricated a part to increase the angle on incidence by offsetting the leading edge of the vertical stabilizer. After installing the part, he took off on a test flight and his partner followed in a chase airplane. The airplane was observed to enter a shallow descent and yaw slightly about its vertical axis, also described as shallow Dutch rolls. The pilot said he had a problem and the airplane impacted terrain shortly thereafter. The fabricated part was never found. A small hole had been drilled through the tube where the leading edge of the vertical stabilizer attaches, but no hole had been drilled through the mating tube. There was evidence that a pin had previously been safetied through this hole. It could not be determined if the pilot had inserted the pin prior to taking off. Load carrying brace wires beneath the horizontal stabilizers were found stretched in tension.