I keep the original finish when it's as nice as on yours. Dull and dirty shellac cleans and polishes up very nicely. There's plenty of very worn machine worthy of a full refinishing. You have to post a picture, your project makes me very interested :- )
The presser bar, needle bar, and the rods under the machine polish up like new by hand. With a product like Quick-Glo on a cloths, it's done in literally two minutes and machines cannot get it any better or quicker. I haven't tried it on the rougher castings yet, but I guess it works there too.