On a pre-04 mille, a can rather thna a whole sytem wont need any fueling sorting. although most milles can benefit from a PC111 and custom map, its when you try a full system that a differnet chip and pc111/c map is required.
Alot of slip on cans like Akra, arrow etc have been designed for the bike so shouldny=t have any fueling issues.
Theres loads of choices out there depend on yer budget. I personally would go for a second hand Akra or arrow system, well made and light(esp the akra).
HTH
cheers Ck