Not a programmer are you? If you customize the engine, it's no longer the same as the original engine. Likewise, Amazon has customized their engine.ITS THE SAME GOD DAMNED ENGINE
All those "numerous customizations and code" was easily ported, BECAUSE ITS THE SAME GOD DAMNED ENGINE.
No one cares if it took 3 minutes or 2 days or whatever else you want to make believe and hope is true. It's done already and working exactly as it should.
Much smaller changes can easily take more than 2 days to complete, never mind all the regression testing that's required.