There's a number of companies making the valve spring compressor to change valve springs with the heads still on. As Bowtie mentioned, you will also need the air connection to screw into the spark plug hole and fill the cylinder with air to hold the valve closed and make it possible to remove the springs without dropping the valve in the cylinder. We broke a valve spring one day at the races, no compressed air available. We pulled the plug on that cylinder, put some rope in the cylinder and turned the engine over by hand to wedge the rope between the piston and the valves. Certainly not the recommended method but it worked on a Sunday afternoon!!! Sometimes ya just gotta improvise.