I think if you are filling in a ring/groove then it's probably Mig rather than Tig that might be best. Don't really think Tig will fill anything like Mig as Mig uses a filler 'rod', just choose a filler metal with similar characteristics to the steel the piston is made from. Ok, sounds easier than it might be to do well but think that's the way I'd go. Hopefully you will get some other advice & be able to choose which way you tackle it.