If you are looking to build something like this:
http://www.aagunsales.com/Reviews/Remington%201100%20Competition%20Master%20and%20Mockup
Here is the parts list to build that exact configuration using all factory Remington parts as replacement parts for the 1100 Competition Master/1100 Tac 4.
M1100 3-Gun Upgrade Parts List
CRITICAL COMPONENTS ARE HIGHLIGHTED AND FEATURE BOLD TEXT
PART # DESCRIPTION PRICE
F300903 MAGAZINE TUBE EXTENSION ASSEMBLY 22" BLACK OXIDE $24.65
F103255 MAGAZINE EXTENSION SWIVEL ASSEMBLY PARKERIZED $31.40
F93463 MAGAZINE EXTENSION BRACKET SCREW $3.85
F99833 MAGAZINE TUBE COUPLING $13.50
F300892 MAGAZINE SPRING, 12 GAUGE, 9 SHOT $3.25
F106090 12 GAUGE FORE-END, BLACK, SYNTHETIC $35.00
F92547 MAGAZINE CAP DETENT SPRING $3.00
F92556 MAGAZINE CAP DETENT TUBE $3.00
F92557 MAGAZINE CAP DETENT $3.00
F202233 1100 OPERATING HANDLE - COMPETITION MASTER $15.00
F300922 1100 FRONT SIGHT ACCESSORY KIT $20.25
F300896 1100 SIDE SADDLE KIT - 7 SHOT $39.50
F240508 1100 BARREL, 12 GAUGE, 22" VENT RIB, REMCHOKE $196.00
F99374 12 GAUGE STOCK ASSEMBLY, BLACK SYNTHETIC $65.00
F300820 R3 RECOIL PAD $35.00
F202050 R3 RECOIL PAD SCREW $6.00
F300895 1100 FRONT SIGHT - COMPETITION MASTER $15.00
F15358 STOCK SWIVEL SCREW $3.60
F94241 STOCK SWIVEL SCREW SPACER $3.00
F300902 1100 COMPETITION MASTER CARRIER ASSEMBLY $29.10
TOTAL PRICE FOR ALL COMPONENTS (TAXES, SHIPPING NOT INCLUDED) $548.10
TOTAL PRICE FOR ALL CRITICAL COMPONENTS (TAXES, SHIPPING NOT INCLUDED) $381.75