Thanks for all the tips. I've spent a few hours researching your suggestions. I've definitely got a direction now.
Well I love the Ford flatty V8, I grew up on them. I've been looking at large V-twins in minibikes and figured.. I can do better. I can slice up a junk block and create a V4.
It would be equal to 1,800cc, that sounds like a darn fun minibike.