I bought a 660 with a twist throttle but I want to convert it back to a push. I bought the push assembly and new throttle cables. Im thinking I need to get new grips? Am I missing anything?

 

Usually the grips just slip on and off (not too easily though) and you can re-use the same one...otherwise grips are not too expensive to buy. Also if you like having the twist throttle sometimes but prefer the thumb throttle most the time there is something called the dual gasser that is really cool and is two in one... here's a link. You can find them on ebay too...
Terry Cable Dual Gasser

 

You can't re-use a grip on the throttle side, the grip is sized larger to fit over the throttle tube for a twist and therefore will properly fit the handlebar without the throttle tube.

You'll need a new grip on the right side but since they come in pairs you might as well replace both.