I'll put it this way: The 1.9 HO uses a slightly smaller intake valve (should fit the 2.0 intake valve just fine with the stock seat), and a hemi combustion chamber. The 2.0 uses a split intake port, but uses a leanburn setup. The pistons, however, should work fine with a hemi head. I suggest the 1.6 head because, while it has very slightly smaller intake ports (1 or 2 mm smaller than the HO ports) the exhaust ports are much, much larger. This is good for high revs. You'll lose a little bit down low, but make up for it up top. Plus the 1.6 has a lot of cam options (aftermarket if you can get away with it), that will really open the engine up. Couple that with the extra displacement, and you'll see where I'm going. Plus the 2.0 uses an 8 counterweight crank, vs. the 4 weight style of the 1.6/1.9. It'll rev more freely.
Stock power out of a roller cam 1.9 HO is 110 hp.
Stock out of a 2.0 SPI is...110 hp. Of course, if you could get the SPI to work on its own, just use the HO's roller cam with a magnet slug in the end for the EDIS system, and you'd make more than 110 hp. But that would require completely redoing the wiring harness. I'm just partial to the 1.6 heads, too
