Are there any manufacturers making a 22mag AR upper?

None that I know of.   There have been a couple of home made ones posted on here.  

I have always thought that would ROCK.  How would one make a home made one ?  I want one !  The re-bored chamber is an obvious,  what about mag conversions?

The magazine is the main problem.  I'd start with a magazine for something like a Marlin or other single-stack mag and either build a box around it like the Ciener mags or make something to hold it into a .223 mag like the M-261 adapters.  

Rechambering wouldn't be a big deal.  You'd also have to move the ejector back to account for the longer case and maybe allow for more bolt travel.  And probably add some mass to the bolt and/or increase the recoil spring to account for the higher pressures.