Yes, the heads are the same.  You can make a 1.9 block a 2.1 by
changing the crankshaft.  (the cylinders are the same, just the
stroke is longer)

However, your machine shop will have to grind a bit off of the
connecting rods (then ensure they are balanced) so they don't hit the
1.9 case (the 2.1 case has a little more room inside).

You may just want to look for a good used 2.1 case and crank and use
your pistons, heads etc.

>Question:  Can the 1.9L engine be upgraded to an essentially 2.1L engine
simply by changing a few parts......like cylinder barrels, pistons, etc.