You can lower the compression several ways. E.g. changing the shape of the pistons, shortening the connecting rods, or changing the heads to have larger combustion chambers. You can also shorten the crankshaft stroke, but that will also lower the displacement of the engine. The point is that it's not going to be cheap and you really need to do this to add a turbo- or super-charger, even one with a low boost, if you want a reasonable combination of reliability, durability, and performance.