Assumming early twinshock. Then its a relatively straight swap, the engine will bolt straight in and exhaust mate up.
You would probabaly be better off getting 600 carbs rather than trying to rejet.
A 600 CDI would help, the UK and import SRX's have slightly different electrics so you will need a CDI of a 1JK import (the UK 1XL CDI can be made to work but its fiddly.
If its a later SRX400 twinshock with round three spoke wheels then the gearbox sprocket has a different offset to the 600