The last of the 3.0 F1 engines were nudging 1000bhp out of 3.0 litres (which equates to about 330bhp per litre), so on that reckoning you can figure the unlimited cash power limit for a n/a 1.4 to be around 460bhp
Seriously though the way to big power out of n/a engines is to get them to rev, however the Micra doesn't have a good enough head design to flow enough air for big power/revs.
The 1.4 we built for our rally car had standard pistons, rods and crank, head was mildly ported but used standard valves and springs, with Tomei cams and ITBs it is 93bhp @ wheels (125bhp @ engine). This was a good budget build and in a light car does really well.
For comparison an ultimate Bowyer Vauxhall 16v 1.4 rally engine (as in Novas and Corsas) are around 210bhp.