550mm = about 21.5"
I figure about 13" from axle to the top of your tire. That leaves 8.5" from top of tire to approx. bottom of headtube to get 550mm. That top of tire to bottom of headtube dimension doesn't look anywhere near 8.5" in the photo. And this is still without sag we're talking about.
I think your 440 replacement fork is probably close enough for government work.