Cessna 421 pilot reported an engine malfunction during climb. The pilot reduced power on the malfunctioning engine then diverted to an alternate airport and landed safely.

Narrative

The mission was a flight from ZZZ1 to ZZZ with 3 souls on board; the pilot and two passengers. Climbing through about 12000 feet MSL heading approximately 090 degrees approximately 30 nm East of ZZZ1; I was cleared directly to ZZZZZ1 for the ZZZZZ STAR into ZZZ. Just after receiving that ATC clearance; my right engine began to run rough. I advised ZZZ Center that I was having problems with my right engine and coordinated a turn back West to the ZZZ1 area. I reduced power on the right (rough running) engine and requested direct to ZZZ2; due to the availability of large runways and crash; fire and recovery services. I [requested priority handling] and was cleared directly to ZZZ2 and told to expect Runway XX. After reducing power on the rough running engine; it smoothed out somewhat and I decided to leave it run. It was providing some small amount of power and gave me redundant electrical and hydraulic sources. I configured and flew a single engine approach to Runway XX; while leaving the right engine running just above idle power. After landing I cleared the runway and then shut down the right engine and taxied into the FBO ramp. Initial inspection of right engine showed the case of the roughness was a crack in the head of the #6 cylinder head allowing all expanding gasses from combustion to escape.