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An important issue is that since you aren't the artist, you don't hold the copyright ( ie, the legal right to copy this work).

Since this illustration dates from early in the 20th century, there is a possibility that it is in the public domain, but in the absence of such status, you are legally and ethically obliged to obtain different versions of  this work (including a higher resolution version) from the estate of the  artist (EH Shepard) or their licensed representatives.  I do know that some of his original illustrations have been recently sold for very large sums of money.

Here's an article about such matters that just appeared:

http://www.wtop.com/?nid=893&sid=3614222

There is also a technical problem with your request.   It can be seen as follows. Divide your 496 px long dimension by the 16.5 inch long dimension of an A3 sheet, and you get 30 pixels per inch.  That is 10x lower resolution than the commonly accepted value of 300 ppi needed for a printed image to be perceived as sharp. Since this 10x ratio also applies (approximately) to the short dimension, it means that you only have about 1% of the number of pixels you need to make a sharp image on A3.  No algorithm ever developed can accurately fill in that many missing pixels.

Tom