Yes and no. While a cast crank is a down grade, there still plenty strong. The low compression pistons are as you say, easily changed out. Even so, a set of closed chambered heads will raise the ratio to a reasonable level. You'll need to know exactly where the piston sits, the head gasket dimensions, valve relief amount and head chamber cc amount for the calculators. Getting to a 9.0-1 should be easy enough. More is better. But lets see where you end up first. At 9.0-1, thats enough for most street cams to perform vcery well.
