If you're willing to machine pistons, why not get an ABD like Tommo did and machine the pistons down in that? The engines are cheap as chips, and all you'd need to do was sort the pistons out.
There's no rare/pricey 1W crank to mess about sorting this way, and if/when it dies you've still got your original G40 motor to throw back in (or build up as a 1.5 1W long-term project).
To get sprightly performance off-boost you need to raise the compression ratio. The 9.3:1 CR you can get with metal HG and 77mm Accralite pistons is very nice off boost compared to a stock G40 - though does need careful mapping as it's very sensitive to ignition timing on boost.