[welcome] to FF! I believe that the Focus didn't have a passenger seat weight sensor until the '05 model year. My '05 ST had one, my '04 SVT doesn't. I have seen for sale, on line, the male portion of the seat belt buckle. You could also get an original version from a salvage yard, probably for less. Either one should "fool" the passenger side air bag into deploying in a frontal collision, although I don't know why you would want that to happen.
My experience with Foci SRS confirms what Bryan says about Foci SRS components being model year specific. I also agree with him that it will be best to return your SRS to OEM specs, as your auto insurance carrier would likely not pay for any collision injuries, and would certainly cancel your policy were they to discover that your SRS was inoperative. Good luck.