Remaking SMW would be a staggering feat because so many enemies and sprites ARE NOT in NSMBWii. It has a lot but not everything. Basic stage layouts wouldn't be hard for the most part but anything sprite related will be tough.
You see, this is where the word "new" comes in.
dictionary.comnew??[noo, nyoo] -er, -est, adverb, noun
–adjective
1. of recent origin, production, purchase, etc.; having but lately come or been brought into being: a new book.
2. of a kind now existing or appearing for the first time; novel: a new concept of the universe.
3. having but lately or but now come into knowledge: a new chemical element.
4. unfamiliar or strange (often fol. by to): ideas new to us; to visit new lands.
5. having but lately come to a place, position, status, etc.: a reception for our new minister.
6. unaccustomed (usually fol. by to): people new to such work.
7. coming or occurring afresh; further; additional: new gains.
8. fresh or unused: to start a new sheet of paper.
9. (of physical or moral qualities) different and better: The vacation made a new man of him.
10. other than the former or the old: a new era; in the New World.
11. being the later or latest of two or more things of the same kind: the New Testament; a new edition of Shakespeare.