Drain mushrooms, reserving liquid; add enough water to reserved liquid to make 1 cup.
In 3-quart saucepan, fry bacon till crisp; drain, reserving drippings.
Crumble bacon; set aside.
Combine sugar, flour, 1/2 teaspoon salt, and 1/8 teaspoon pepper; blend into reserved drippings in saucepan.
Combine vinegar and reserved 1 cup liquid.
Add to flour mixture; cook and stir till thickened and bubbly.
Meanwhile, cook macaroni following package directions; drain.
In saucepan, lightly toss macaroni with dressing, mushrooms, bacon, onion, and celery.
Spoon into serving dish.
Garnish with radishes and parsley, if desired.