Tintypes are rare in that they were not produced from negatives. When you look at a tintype you are looking at a single, one-of-a-kind photo image. For this reason tintypes should never be worked on directly because of the risk of compromising their historical integrity. Today the accepted process for restoring tintypes is to do it digitally.
Study the surface of the tintype and make sure it looks stable with no bits of emulsion or rust flaking off. Take a cotton swab dipped in distilled water and squeezed dry and carefully clean the surface. Use a blow dryer set on low to gently dry the surface.
Set the tintype so it's well lit and use a digital camera to take a picture of it. Set the camera to use the highest pixel level and shoot the image as a bitmap.
Import the image into a photo imaging program and immediately save it using the "Save As" command--don't work directly on the original picture file. Digitally clean up the image if there are soiled areas: adjust contrast, brightness and saturation, color, etc.
Print a good quality laser or inkjet photograph of the tintype and have it professionally framed.