Pilot reported engine failure due to a crankshaft failure caused by improper engine overhaul procedures. The pilot performed a diversion and immediate precautionary landing.

Narrative

On Date I hired Company at ZZZ1; to conduct a Top Overhaul on Aircraft X. Company is a FAA Part 145 Repair Station; Air Agency Certificate number X. On Date1 I picked up the aircraft after the top overhaul was completed. On this day I was met by a young man; who was not a certified A&P mechanic; who claimed that he had conducted the top overhaul of my engine by himself. Further questioning of this individual caused me some concern because when I spoke with him briefly he did not seem to be a very knowledgeable mechanic. During the course of discovery it was determined this may have been a Name who was a non-certified mechanic.I then immediately spoke with a Name1 who was the owner/operator of Company about the repair work done to my aircraft. Name1 assured me that he had personally conducted the top overhaul on my aircraft engine; that all work was completed properly; and that the young man was just bragging. I had the word of an experienced IA (Inspection Authority); and a signed endorsement from Name1 stating that the work had been completed IAW (In Accordance With) FARs; and that the Repair Station Manual requirements were complied with; so I paid Name1 invoice for the work and left.On Date2 I was flying over ZZZ which is a densely populated area. I heard a bang from the engine compartment and observed a drop of about 300 RPM on the engine. There were no abnormal vibrations; and all the engine gauges were in the green. A visual check of the air frame showed no abnormal conditions. I thought that I had a broken magneto; so I started to fly directly to my home base of ZZZ2. By the time I was about 5 miles from ZZZ2 all of the engine gauges were in the red except for oil pressure; and the engine RPM was dropping. I landed and shut off the engine once I cleared the runway and taxi ways.The next two days of trouble shooting the engine revealed no visible signs of damage; and the cause of the power loss could not be determined. On Date3 the engine was taken to Company1 at ZZZ4 which is a FAA Part 145 Repair Station. Upon tear down of the engine it was discovered that the main bearing was crushed which caused the crankshaft to break into two pieces. The metal debris from the broken crankshaft had gone throughout the engine making most of the engine parts unairworthy. The owner/operator of the repair station; a Name2; stated to me that the cause of this failure was the over torquing of the cylinders during the top overhaul at Company. Name2 called Name1 on my behalf in my presence by telephone.The telephone call that was done in my presence; and was very brief; the top overhaul was improperly done; and what would be his course of action for correcting this situation with me. Name2 hung up the phone with Name1 very quickly and told me that Name1 was not going to do anything about the damage to my aircraft. For that reason I then proceeded to have Name2 order new parts and have him rebuild the engine. Name2 failed to submit a Service Difficulty Report which I believe is required by FARs 145.221 (a)(b) (1-7).Fortunately my incident; caused by the improper repair of my engine was not fatal. No one was injured when we landed with loss of forward thrust. However; the next pilot may not have the good fortune that we had.