If its going to be a machine used regularly I would most definitely go with the new briggs engine or even perhaps try to locate a honda G100 (i have one of these on mine and its great it came off an edger of all things) the old kirbys while not too bad in their day are getting hard to get parts for and are quite old now.
Everyone has their own rust removal techniques, I'm limited on space so just use an old wire wheel in a drill and rust converter, some may use an electrolysis bath, molasses, etc.