Bigger engine, the 450 does well in an ATV but it has too be at least half the weight of a defender. I squeezed a bit more power out of my 450 outlander with an EJK fuel tuner and HMF Titan Qs slip on. Besides that maybe put some more miles on and see if it untaps a little more. Maybe a clutch kit if there is one available. Besides those three mods there isn't much that can be done without opening up the engine and experimenting with half big bore kits.