As far as I know ZXR400 is like most older inline 4s, as Vimtozz says, engine out, turn it up side down, casings and clutch off and then split the crankcases, the bearing is usually around the 30 mark, but it's the labout that makes the job expensive.
Parts availability should be good, try Cradley Kawasaki for bits, if they haven't got what you want try CMSNL, but their postage is expensive.