Welcome to the forum.
Do you already have an idea of what sort of modifications and/or accessories you want to add during the early days of your build?
This will be a track bike. I am a big guy 6'2" 225 lbs so of course suspension will be first on the list when its available more than likely Ohlins cartridges for the front and Ohlins shock for the rear. Engine I will leave stock for now. Remove head lights, all extra unneeded weight. Install a flasher on the rear break light for races in the rain.
I have a few items that I am pretty sure will fit from my N300
Shorty clutch/brake lever
Nissan master cylinder from an 04 GSXR
Quick Shifter
Attack rear set
Hoping these will fit
Brembo 4 piston caliper
DID 520 chain with -1 front and +4 rear sprocket
Ohlins rear shock.... A big maybe
N300 race fairings I doubt it but maybe something might I will know soon
Things that wont fit will go up for sale
ohlins 37mm cartridges
Stock fairings - used for 3 days
race fairings if nothing fits to the 400
still waiting for the N400 to get to the Philippines