In the US, if operating part 91, the engine can be run on condition for as long as you are able to pass an annual inspection. Unless there is an FAA approved airworthinesss limitation on a part or assembly or an airworthiness directive that imposes a life limit on something, nothing is mandatory beyond an annual inspection. Service bulletins, instructions or other manufacturers information can only be recommended unless associated with an AD that makes them mandatory. Your insurance carrier may not allow you to avoid compliance with a so called " mandatory "service bulletin" but it's not a violation of the CFRs. Outside the annual or other inspection program you may use as an annual inspection substitute no other inspection or overhaul is required under part 91. Life limits in chapter 4 of most maintenance manuals is the only section that is actually FAA approved ( and signed by a Fed. on the document) in the maintenance manual.
As for me, I wouldn't fly in a ship that had not undergone all of the manufacturers recommended inspections and overhauls and part 135 normally makes them all mandatory but I also wouldn't bother arguing with an owner who doesn't do them if they understand the regulations. The engine manufacturers time in service and calendar time overhauls are recommendations only, not mandatory unless parts or assemblies are listed as airworthiness limitations.