Either that or screamers. The first option would be to fire 2 cylinders at a time. But with 8 that isn't that big of a problem since you have a fire every half of the rotation while MotoGPs big banger would be on every rotation. A screamer would have 8 independent fires, making it 4 per crank. So, what is it then?
EDIT: since the size is a constraint, it's the most easy way to put two cylinders togheter on the crankshaft, making the engine a big banger.