97 XR400

I just pulled this out of a friends garage. It's been sitting for 6 yrs without a start. He said it ran fine before parking it. I tried kicking it but feels locked up, wondering if the piston is corroded to cylinder from sitting so long. Besides draining changing fluids, is there anything I could maybe pour on top of cylinder to break it loose just to get it to crank? Bike is bone stock with low hrs and looks like a fun bike to get running!