The following Action Replay codes will allow you to use the alternate costumes found in 2P mode in the normal game

PAL VERSION

Press A+X to use alternate skin, press A+Y to not use:
T51B-HQ07-BM52H
J7H3-6WVJ-4QXM4
10TM-3CKU-NB64W
MXAY-9KPE-MDVAA
24ZN-ZTWU-Z04D2
QRF1-8CM2-GDBTN
4XUH-Q94T-7VKMR
7QPR-A3RU-9D9QJ

Knuckles Fix (Will disable air neclace and sunglasses)
K9C5-XA0Y-H473D
QTE3-6P9D-V1R46
9NE0-HK16-BXCMU
YC9N-HE0R-QWFDC

Tails Fix (Will disable any upgrades)
2B8K-2V24-4H1TF
D501-3BE1-25J24
QPUH-VFVZ-GVFM5
EDEJ-J39J-8HRTP

Eggman Fix (WIll disable any upgrades and, like in 2P mode, he cannot aim)
FNXK-UJME-U2HJR
NMBT-ZR20-X4F2N
971H-N591-292P7
YEVM-U9G9-C9P4E

NTSC VERSION

Costume Enabler
UUKC-MUAU-1UVJN
AHUJ-AQXH-P5189

Sonic Costume
C316-BX71-JQF59
FRGD-QZCF-7K7PG

Knuckles Costume (Requires Fix Code)
BKET-PFFM-MAC8V
VRAM-866Z-Y324J

Rouge Costume
TG1U-CQA0-TGHZP
WW47-M1XA-QJCD5

Tails Costume (Requires Fix Code)
424C-J1B4-VG21N
3WKF-DYJK-YV4T3

Robotnik Costume
7Q93-MJN9-D8X27
J301-G0QT-3P764

Knuckles Fix (Will disable all upgrades)
W19V-BFPR-RDE3Y
U091-2RCT-GQJE5
ED04-HN9H-A6FT7

Tails Fix (Will disable all upgrades)
ZXBY-1D3H-Z5HJD
PHDM-2TD7-0MYTB
QQ06-R6BD-UZQNW

Robotnik Fix (WIll disable all upgrades and, like in 2P mode, he cannot aim)
VWCK-PNMY-3N87W
3JFH-97G6-NRCXK
HC1U-X4MH-QPN0G