About 180bhp is about the limit afaik, but we're talking alot of money to get it to that kinda power.

I paid £250 iirc for a 1660 with high-lift cam. I've spent a few quid on it since and it's amost 100% reliable and does make a nice sound and goes quite well. Obviously if i fitted twin carbs i should be seeing about the 130 region which would be ideal. Except it already does 15mpg so i just couldn't afford to run twin carbs

Unless you really want to i would go zetec, run it on carbs with edis ford ignition or megaspark and with a few choice mods should see 140-150 with ease.