i'm guessing here (so the info is worth what you pay for it) but i presume your car is cable throttle. so, you need to find a larger throttle body that is also cable throttle. according to the link below, the 1999 996 c2 throttle body is larger inside diameter, same outside diameter, bolts to the 2.5 plenum, and is cable operated. the intake duct should be direct connect also, unless the 996 throttle body is physically longer.
Bigger throttle body
should be plug and play. best to find the used part someplace, however, as Auto Atlanta is charging $1,366!