I’m a bit late on this, but the excellent Blog of Holding has published stripped down rules for Spelljammer, trying to fit the essentials onto one page. I agree with his point that Spelljammer has a couple cool ideas, then bogs them down with a bunch of overly-complicated rules about Grubbian physics and extra rules for clerics. To quote: “I’ll break out my copy of Spelljammer. OH NO IT’S 200 PAGES! THIS BOOK IS TAKING TOO LONG TO READ! THE PCS HAVE ALREADY IMPLODED IN THE VACUUM”
The first blog post covers the basics of the setting and physics, boiling it down to a short paragraph (plus expositions explaining the decisions to help the DM understand why they have done, something useful for when you want to expand upon it for your game.
The second builds a 20 entry random encounter table that also helps explain the setting.
And finally they flesh out the setting and compile things into a one page (illustrated!) setting document.