Pilot flying SR20 aircraft reported engine failure in cruise. Diverted and landed uneventfully.

Narrative

Planned IFR flight from ZZZ to ZZZ1. Uneventful departure and climb to cruise altitude. Upon level off and established enroute power setting; attempted to lean engine. Moved engine mixture less than a knob-width and all cyclinder EGTs went to zero with 0 fuel flow; observed what appeared to be reduced power and knocking noises from engine. Unable to maintain altitude and advised ATC of partial power loss and requested 5000 ft. (given). Initially stayed on course and attempted unsuccessfully to restart engine with throttle and mixture; and propeller still turning at approx 65% power. Descending thru 5000 ft. requested and given 3000 ft. and divert to ZZZ2. On reaching 3000 ft. on ATC vector to ZZZ2; ceased restart attempts; and requested priority into ZZZ2. Directed to switch to Tower; informed tower of power loss and given Runway XX landing clearance. Power appeared to continue to decay in pattern turn; completed 50% flap dead-stick to uneventful landing. Prop stopped on rollout; partially cleared Runway and hand-towed past hold short line in coordination with Tower; followed by tug pull to park.Suspect either fuel vapor lock or engine-driven pump failure during leaning procedure; fuel boost pump was not on. I did not declare an emergency and should have both done so and asked for an earlier divert. With sufficient altitude to execute a power-off landing at ZZZ2; use of the aircraft's chute was an option but not elected at 1000 ft. in favor of landing.