Welcome to the forum and for restoring a 400/4. You are lucky as most parts are available for these either new or secondhand.
Be careful if you choose to fit taper roller headset bearings though as a lot of the kits do not leave enough thread on the upper yoke centre nut. Having learned this lesson twice over I'd fit loose ball bearing kits now.
I would also always replace the OEM plastic swing arm bushes with metal items as well, Nurse Julie sells the kits on here and she offers a discount to forum members. Check the clutch basket cush drive rubbers as a must, they go rock hard and split making the the initial take up from pulling away very rough. You'll find a thread on here detailing how to check and change them, replacement rubbers are available from Max.
Lastly, you may not be aware of this but apparently the yellow ones were always the fastest although that could just be a rumour spread by myself 😉
