wouldn't be the first time I put a piece of wood on a piston and whacked it to get one moving.
at this point.. you don't have much to loose.. ;) you are starting with a broke engine.. ;)
if the pistons are aluminum.. discount this.. but otherwise a lil clr/naval jelly or iron eating driveway cleaner poured in for a bit WILL eat the rust.
past that.. the loosen juice cocktails and a lil grunt work usually prevail..
keep us posted.