Here we have drawings of brackets for the air conditioning compressor for the MGB. Until proven otherwise, I suspect they would work as well for the MGA. These drawings were (reportedly) done from dimensions taken from an original dealer installed AC as specified by BMC. This copy of the drawings was supplied by Charles Durning . Creator of the drawings is noted on the drawings as "© Simon Lacey 2006".

With a bit of research I found this on a BBS. Simon Lacey pulled the original AC parts from a 1980 MGB LE Tourer in a bone yard at Ecology Auto Parts in Chula Vista. The compressor was a Sankyo SD-508 (but looks VERY similar to the "Sanden" SD-700 on his 92 Range Rover). Seems like they are the same company and the Rover unit seems to have the same fixing points and "piggy back" mounting for the MG alternator. You could tie up an ocean liner to them! As is common, the BBS archive does not have dates or places or contact information for the poster(s). However, Simon did post a link to his web site where this information is posted. Try this link:
http://www.simonlacey.com/pages_new/content/mg_link_page.htm
There are also some nice pictures of the brackets.

That web site has since been deleted, but here is an archive copy of the page from April 2017:
http://web.archive.org/web/20170423154448/http://www.simonlacey.com/pages_new/content/mg_link_page.htm
Unfortunately the information page linked from that home page is not archived, so what you find on this page is all the information I have available.

Addendum, October 18, 2020:
Good news, I have recovered a copy of the brcket drawings from Charles Durning, and have posted this on my web site, so the image links above now work again.