Having owned an S800 I couldn't imagine a modern motor in one unless it was the size of a K3 which is a god-awful lump compared to the roller bearing original motor. B series motors are out as they rotate the wrong way (S series cars are RWD). F20C is being done in the US and will require some cutting, but I have a feeling getting it engineered in Australia will be difficult (displacement -> weight calculation anyone?) http://honda-tech.com/showthread.php?t=2399761

4AGE does fit but will need a big brake upgrade. Cheap, tough little motor that will dump out decent power (and could later switch to 4AGZE). I'd be going down this path.

Do you know if your 600 is chain drive or converted to live axle? S800s had the live axle but 600s ran chains through an oil bath to power the rear wheels - you need to factor that into your conversion.

It's been done as a track car already http://www.rollaclub.com/board/topic...to-honda-s600/