It is fine to do that. I would polish the new carbon steel part to match the stainless steel color and finish. If polished smooth it will not rust easily as rust likes to start in rougher, textured surfaces. I have successfully polished blued screws to match the originals on stainless and nickeled guns when unavailable. Eventually if I come across the correct screw I replace it but the polished screw is really undetectable if done right.