140hp is 100hp per litre.

i would use the A14 head as it has bigger ports, but the A12GX head also has some nice bits about it too - like higher CR.

i would use injection to make it reliable in the upper RPM, you'll need to lighten the whole rotating assembly (like knife edging the crank and light flywheel), use quality HE pistons, bigger wrist pins, a n-n-nasty cam, double valve springs etc etc.

you may even want to invest in roller rockers, but they're pretty expensive.

or, for the same outlay you could have a near stock engine and a small turbo hanging off the side and close on 180 - 200hp with injection and intercooling.