this is worth a read

http://www.pumaracing.co.uk/CVH.htm

and this

http://www.burtonpower.com/technical_1/cvh.aspx

but I would go zetec or duratec if desprate to change your engine.

The cheapest and fastest route to a bit more torque would be get a "high torque" cam from piper or kent, (as CVH cams wear quickly anyway), a vernier cam pully and a decent exhaust system, go for a 4 into 2 into 1, NOT a 4 into 1.

Then take it to the rolling road and ask them to set it up for torque rather than peak power.