Oh, fun. But not so horrible. Assuming they're not point clouds.
Stl files can make a decent solid (sometimes), which can then be used to make a decent parametric part. Been down that road. Sadly, it takes building a whole new part using the Stl as a guideline rather than using it as an actual basis for a part. Can still be done though.
Do you have any original drawings? A layout with intended angles and spacing for things like bores, lifters, and cam tunnel height could help guide things versus just trying to reverse engineer existing parts, which usually winds up at least a little off the mark..
