These already have a 12V dc outlet, but it is only 8Amp max and I expect that it is from a coil 'Tapped' into the 240 coils? .
That's typical, yes.
The Alternators are rated for 650W continuous, 720W max and 950w peak (running from the engine at either 1800 or 3600 rpm presumably). So any Ideas what kind of output they would have if rewound to 12V
Exactly the same wattage. Current will be multiplied by 20, voltage divided ditto. (Somebody else can tell you what RMS voltage you'll actually want for your application - it won't be exactly 12, so the factor won't be exactly 20.)
To rewind them, unwind the output coils, counting the turns. Rewind them with 1/20th (or whatever) the number of turns using wire with 20 times the cross-section.
Of course that would mean you're winding with bar stock and you'd have extra eddy-current losses from the increased wire thickness. So you'll want to do with something considerably less than 20 times as thick going several-in-hand so the product of the thickness increase and the number of paralleled strands is 20.
In principle you could wind 20-in-hand with the original wire, making 20 paralleled coils each with 1/20th the number of turns of the original. In practice the original wire will probably be too dinged up to use, and 20-in-hand is a bitch. So you'll want to use fewer in hand of something somewhat thicker. Probably the thickest wire you can easily get to fit the slots and bend into shape when winding.
Wire cross-section doubles for a count-of-three decrease in wire number, i.e. #16 is four times the cross-section of #22.