There's nothing special to making them look "original" and it won't mean anything for resale, either. It's just a plan cushion - nothing special.
Of more importance, is to make them suit what YOU want. Cut a piece of cardboard to the size you want as a template and then get sewing! Take note to where the snaps are... snaps on a horizontal surface dictates installing a piece of vinyl across the corner underneath so you can reach your fingers in there and snap the snap. Snaps on the front, vertical surface would dictate a flap along the front edge of the cushion to install snaps into.
Same goes for the bow cushion - make it whatever size you want. Make it to fit where the current snaps are or install snaps in different locations and make the cushion bigger/smaller.
Reach out to NorC... if those cushions are the proper size, that seems like a good direction to go. You have to move the snaps on either the cushion or the boat - but that's easy enough.
EDIT: My last paragraph was supposed to say "Reach out TO NorC", not "Reach out NorC"... I fixed it.