The name 2.1 is a bit misleading, if you look at the actual capacity it's 2.045 so less than a 2.5% increase in capacity.

If it's a used block get it bored, the walls between bores are narrow but it tends not to cause any problems. I would only fit standard pistons to a brand new block. With a re-bored block at least you know you've got spotless bore to start with (as long as it's done properly).