the only way to remove the race mod is via a gameshark pro or on-comp program.
the how to
look up rogsR34uk(?) in the member list. this person has a "play with" site that includes all three US NTSC versions AND the PAL versions.
after locating the silvia serial number in question (each car body is coded separately) look for the part of the code that uses the hex 1x, 5x, 9x, or Dx. the race mod uses "C", and sometimes "D" for the race mod body. change that back to "8" to remove the race mod.