I own a cessna 1966 172 with a 0300-D continental. My mechanic inspected the engine at its annual inspection and found two cracks. One above the number 1 cyl. and the other above the number 3 cyl. These case cracks are addressed in a continental service bulletin that suggests cleaning and marking the crack, then flying for 50 hours and re-inspecting. This has been done and the cracks are progessing past the 2 inch max. I contacted teledyne continental and they no longer support this engine and the area could not be welded.The engine is a mattuck rebuild with 238 hour smoh and 2997 TT.The engine rebuilder that I spoke with, estimated 8,000 to 9,000 for exchanging the case. I have a 0300-C case that can be used but it will still have to be inspected with no guaranties of cracks later. Any suggestions?