what sprocket set up will yield max torque and will retain a 90mph top end? Prefer to go stock front and dif size rear but will not mind changin front and rear sprockets to achieve this.

41 tooth minimum.

Maybe 15 front and 41 rear. Lowering the gearing will increase the "hitting the wall" rpm so hard to say. If you only make 10,500 with stock gearing... lower the gearing with a 15/41 combo and you will make 11,000. Also the top speed is made in 5th gear not 6th. You could easily have a case where your 6th is now low enough to be just taller than the old 5th, and hence make a ton more speed after lower gearing.
Cool.
Srinath.