You might call it M20 Spelljammer. Please don't.
The thing is that no part of SpellJammer is or was Open Game Content. The d20 mini-game in Polyhedron 151 is great, but as stated on the first page, NOT open game content. 100% proprietary. Andy Collins' site has references to material left out of the Polyhedron article, but all of that is also NOT open game content.
The name Spelljammer itself is a registered trademark of Wizards of the Coast. This post will not challenge that trademark. Other items of the setting are also trademarked and proprietary. I say this because SpellJammer cannot be used to reference the idea of magical spacefaring adventure despite the similarity in concepts.
As such, let me begin by saying I want to extend the M20 ruleset (including standard races and classes) to an extra planetary setting. The working title of this setting will be Aetherflow, abbreviated AF. I will attempt to write this from scratch using other M20 materials developed by others as well as my own ideas.