Obviously you've hinted you want to stay with the CVH, but if you can find a 1.8/2.0 Zetec then thats a fairly straight forward conversion and will get you a solid increase with far more tuning capability (in both N/A and turbo form). Silvertop is less messing around but they are getting a bit thin on the ground these days.

You could definitely source an entire Zetec engine for less than a proper gasflow job tho - so something to think about