The pilot departed Louisville, KY reportedly with a known oil leak in the right engine and was on the second leg of an on-demand air taxi cargo flight. About 14 minutes after entering Mansfield Approach Control airspace, the pilot requested and received an altitude change from 5,000 to 3,000 ft because he 'was picking up a lot of ice.' After entering Cleveland Approach airspace, he reported the right engine had failed and requested to return to Mansfield. The pilot was informed of Mansfield weather. He then indicated he wanted to try Cleveland, then reported he could not maintain altitude and wanted to go to Mansfield. The pilot was receiving vectors from Mansfield for an ASR approach to runway 23 and at about 1 1/2 miles from the threshold the pilot reported he was lowering the landing gear. The aircraft then disappeared from the radar scope. Investigation revealed improper weld repairs to the right engine case and separation of the number six cylinder from the case due to fatigue cracking in the through bolts and studs.