LET pistons bring the CR down a little too much - good for Zetec Turbos. C20XE (redtop 2l) pistons will get you down to about 7.8:1 on a 55cc CVH head. You will need to machine the little ends for these so it will start getting more expensive. Oilways also need to be drilled in the pistons as the Vauxhall pin floated in the little end, unlike Zetec pistons.

4 oil returns need blocking, water pump needs a plate (ebay), and engine mounts need some modifying.

There is more but that is the basics. Check the Jim Hearne Quantum site for more details.