:headhurt: Simple part, tough solution. Best bet would be sourcing one from another Hitachi, one that's only good as a parts donor. Those, are rare as rocking horse s**t :horse:
Those springs are highly specific. You might try a good hardware store, that sells miscellaneous pieces in bulk; if lottery-winner luck is with you, they might have what you need. Problem is, just being a little off can make a huge difference. Slightly thicker wire, different material, different diameter, even one coil plus/minus is all it takes, then you just plain have the wrong spring. Maybe searching the web for a specialty supplier. You'd likely have to send them the one you have, to make a match possible.
Honda never sold flywheel parts separately, the entire flywheel assembly was sold as a complete, non-serviceable, assembly.