[JP Noonan]

The main problem with 2.0L heads is the angle of the spark plug hole increases the chance of a heat stress crack forming between the plug hole and the exhaust port. Once that happens the interference fit between the seat and port is lost and the seat drops.

The 1.7-1.8L don't have as much a problem but then the also don't benifit from the port design and spark plug angle of the 2.0L

There is more to it than just valve size, but 1.8L heads with 2.0L size valves and a big bore kit is going to be a nice motor. With stock 1.8L heads and 2.0L displacement you could bump up the compression ratio a tad to get more power. The biggest thing to get right is to set the whole system up to work together, the cam must match the induction, the induction matches the heads and displacement, etc. Don't put 48mm Webbers on a car with a FI cam and 1.7L heads and expect to go any faster. Talk to the cam manufacture to see what they suggest.

FWIW