In principle you can use the 16V head on your existing 8V block, put your pistons need 4 cutouts for the 4 valves. On my 8V engine there were only 2 cutouts, so this wouldn't have worked for me. Complete 1.4 16v engines are cheap, so no need to change heads.

Exhaust and inlet manifold needs to be 16V to and a 16v ecu is at leat advised. I ran my 16v spi for a whole while with the 8v ecu and never had any problems.