Can anyone recommend what battery to go for for my zx12r powered bec

Unless you have an electric reverse then probably best stick to a standard bike battery from a ZX12R. You'll probably want to keep it topped up with an Optimate (or similar) trickle charger as well.

The electrical loads on a kit car are little different to a bike so wouldnt worry about needing to fit a bigger battery

Yup. Stick to the standard one. With an electric reverse you might want to look at a deep discharge battery....