Could you have the fork milled (or remilled) to take a real ISO crown race? My LBS did that for me years ago so I could use an ISO headset on a Bridgestone that came JIS. It took 5 minutes and cost $5.
I expect the cracked race will be ok. I rode with a cracked race for who-knows-how-long a few years ago because I didn't discover it until I gave the bike it's annual overhaul.
BTW, some current FSA headsets come with cracked crown races (they call them "split") but they are cartridge bearing units and the balls don't roll on the race itself. Sure makes for easy installation and removal.