you can reuse those spark plugs you know. just take a lighter to the tip and clean it off w/ a brass brush or toothbrush or something.
also, if you're plug is wet and black after an idle or low throttle then it's not your main jet that's reading rich, it's your pilot jet. try turning your air screw out 1 turn. actually, turn it in and count how many turns it takes to seat lightly. that will tell you where it's set at now. then turn it out 1 turn past where it was.
even w/ the airbox lid off, the idle circuit will still be rich unless you adjust the air screw.
also, if you do need to rejet the main jet, you will probably have 4 phillips head bolts under the bowl. this will pull off the bowl and you can get to the main and idle jets.
my guess will have to be that your original jug was ported and when you put on the new jug it wasn't. this would change your jetting a little.
also, before doin any of this, try jumping your tors wires that were on the throttle assy. you might need to close that circuit to get the bike to rev.