Probably SimLab’s STEP importer. Import with units set to meters to avoid the tiny face issue. Plan to spend a fair amount of time cleaning up the model, exploding components, and so on. I think the last three or four times I had to work with step files it would probably have been easier and faster for me to model the things from scratch in SketchUp. The last one was a part for a hydraulic braking system on a truck and that was a real pain.