KRC Japan widebody kit adds the KRC rear spoiler automatically, but has unadjustable rear aero, and you cannot apply another rear wing as it locks out the option in Forza Motorsport. Additionally, the KRC spoiler is not available as a normal rear wing option.
In Forza Horizon 5, you can apply any rear wing option with the widebody kit and KRC Japan rear wing is classified as Race, so it is adjustable.