| motor bolts straight in or small mod if using small bolt frame
will need 998 header pipes
get some 999 throttle bodies with injectors shower type.lightly modify throttle pot to read correct voltage and cable
get 998 airbox
slight mod to fuel lines as 999 uses one line with a t piece..
wiring is self explanatory will need to use a 916 style chip as 999 uses single injector and pc3 to map the fuel as it will be so far out as the 999 uses iwp069 injectors wich flow much more than greens.
there are a few things i forgot to mention but nothing u couldnt crack if your mechanically minded | 
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