Are you joking? If it is the engine in a production car, then the engine is in production. It is a production engine. Whether or not the engine is based off of an existing engine is irrelevant.
To you maybe, My point is that most of not all of those engines are not produced by these small supercar makers but modified versions of MASS produced engines, some I bet are nothing more than putting some form of Forced induction on a crate engine and some ECU tuning. They way I look at it those engines are more like tuner work than OEM level work but that could be just me. That doesn't take away from these cool supercars but way different from what a OEM has to do the put a engine into production.
Porsche 918 Spyder 795 hp
Porsche 918 Spyder V8 makes 608 HP
2007 Gumpert Apollo S AUDI V8
2005 Saleen S7 TT Ford 351W
2013 HTT Pléthore LC-750 GM LS7
2013 Gumpert Apollo S Audi V8
2006 Koenigsegg CCX Ford Modular V8
McLaren P1 Nissan VRH35
2013 Koenigsegg Agera Ford Modular V8
2008 Koenigsegg CCXR Ford Modular V8
2013 Koenigsegg Agera S Ford Modular V8
2007 Zenvo ST1 GM LS7
2011 Koenigsegg Agera R Ford Modular V8
2013 Hennessey Venom GT GM LSx