first thing ,bigger wheels and stock st gearing ,if u change to bigger wheels you've affectively changed the the final drive so what u'll end up with is no take off and a great top speed [if the engine is up to it ].
now the next thing is u cant just cut the manifold because this will change the harmonics of the exhaust ,or waves [ pulses back from the exhaust into the engine].
short exhaust = higher top end
longer exhaust =bottom end more torque [take off]
as for the rear dampers time to clean em out and refresh , maybe new seals. but check all the pivotable parts around the swing arms and the shock ,and the hub
hope this helps.