The 1.6 is superior to the 1.8, to start with it's a light alloy engine and 1.6 and 1.8 acceleration is exactly the same. On both the mentioned engines the exhaust manifold is on opposing sides so will not simply bolt on.

Have you considered tuning the 1.6 for it has alot of tuning potential the 1.7 (Puma) saw 220HP in junior WRC.

Martin