adjusting a ready made shorts pattern with front pleats for large stomach

That's a big amount of adjustment, mollybrown!  The easiest thing to do would be to get a pattern in the right size.  Some American patterns have several sizes printed on each pattern piece.  It shows where length and width is added.

To begin, measure the hips and waist of your "client" noting on your tape measure how much is in the back half and how much will go across the stomach, the length from waist to the seat of a chair while your model is sitting on a firm rather than upholstered chair, the whole of the crotch length from center of the front at waistline to middle of the back at waistline and note on your tape measure where the crotch seam would be, length of shorts, and width of thigh.  Compare these to the measurements on your pattern envelope and you will see how much you need to add. 

Trace a copy of the original pattern onto paper.  I use a roll of vellum paper which is translucent but strong.  I order it through Amazon.com.  Include all markings on your traced pattern.

You have two legs:  the hip and waist measurements will be divided in two.  Mark where the side seam of the waistband is located.  Draw and cut a straight line right down the middle of the waistband piece parallel to the straight of grain.  This is located in the center of that part of the pattern piece sectioned off for your front waist and the back waist.  Usually a waistband pattern shows only half the waistband with the intention of cutting it on a fold to get the whole thing.  Spread your waistband pattern sections apart as much as you think you'll need to add.  I use pins stuck straight into my cutting board.  Weights or tape work also.Follow the same procedure if your pattern has a facing instead of a waistband.  Now retrace the waistband pattern including the extra width.

Do the same to the back and front leg pattern pieces.  Adjust for length as usual (indicated on the commericial pattern).  Having noticed whether the most fullness is in the stomach area or the seat you will add more fullness where it is needed.  The waistband alteration used only the waistmeasurement; now you are using the waist, hip and thigh measure.  Draw the vertical line keeping it on the straight of grain.  Keep your inner leg piece on the straight grain.  You might need to angle the hip pieces to allow more room at the hip or whatever.  If you can't get it to look right it's okay to mark a line at right angles to the first and cut down it right to the seamline.  Then you can either spread or overlap it to get the desired contour.  Once you have added about the right amount check the thigh measurement of the pant leg.  Add the back to the front (minus seam allowance).  It has to be at least as much as the wearer's plus some ease.  Use pins or something to keep the pieces from moving, trace the finished pattern pieces.  To fit the crotch, "scoop out" the curve deeper for a larger size.  Roughly copy the curve in the original pattern.  Pockets, belt tabs and other details may not even be affected by the modifications.  You might want to increase a patch pocket by a small amount to look right in a larger size.

Now you can check the larger pattern pieces you got with your second tracing and compare them to the original purchased ones.  If no huge error is visible, you can put those away.

Now this is quite a large amount of change, so you will want to stitch up a trial copy in muslin or any other cheap fabric you don't mind discarding.  Use a large machine stitch or hand basting.  You don't need a zipper or buttons.  If an elastic waist is used mark (don't cut yet) the amount of the original waistline plus the added.  Try on the elastic and see if you have the right amount to get the shorts off and on while still holding them up.  Don't cut the elastic yet until the whole thing is fitted correctly.  Use a safety pin to hold it until stitching.

Do the shorts fit?  Make any changes.  Pin a fold where you have too much fabric.  Remove the trial garment and cut a slash where you need more.  Try on and pin a piece of scrap fabric to add.  Are there lines where the fabric pulls?  Most sewing books show what to do when this shows up in making pants.  Experiment smoothing the fabric from side to side.  Then from up to down.  Which way does it seem smoother?  That shows whether room must be added in width or length.  When the test shorts fit well, rip the pieces apart and change the paper pattern.

As you can see it is a lot of work; but it can be done.