Hi xjqkz, you have a couple of issues to deal with here.
1. Carb. To be honest I would spend the little extra and get a complete new GX Series carb. They are far superior in tuning and performance.
2. This all depends on what Generation your 3 Port is. If it is a Gen 5.5 (with the flames on the cooling head and a black crankcase, then you should be OK.
The new PRS sets contain the 21-0600 con rod, which will only fit the newer cranks with the larger diameter crank pin (Gen 5.5, GX and MG66).
If your motor is the older Gen 5 version with the plain head and a grey coloured crankcase, then the new PRS set
will not be OK - unless you get an older 28-0600 con rod and change it over, or fit a new GX Series TiNi coated crank to your motor to accept the newer con rod.
As far as putting the 5 Port PRS into the existing crankcase, all PRS and crankcases from Gen 5 onwards will interchange. You just have to be aware of the different con rods and crank pin sizes from Gen 5.5 onwards.
Hope this isn't too confusing, but I would hate you to buy something that wasn't compatible
