Install a clutch on the 90 ? Sorry ... never heard of it , or seen it .
And as far as trade ? You'd be better off selling the 90 and trying to get yourself a Zen . Those that have them , realize , that GAS is a LOT cheaper than nitro AND , in general , gassers are a LOT more dependable than nitros ( and I OWN a LOT of nitros ..
)
Also , there is a LOT more you can do with a ZEN ( modding wise ) than you can a nitro .
Again , IMO - try selling the 90 and go from there .