M+GUI source is not there, but MAMEMPlus one is there, just a bit hard to find:
Develop > What's happening? > Click on the revision number
now on the top of the page, if you click on the second mameplus link of the bar
"SourceForge.net > Find Software > mameplus > SCM Repositories > mameplus "
you get to the root of the svn repository and you can browse both mamep and mamepgui sources (not the M+GUI, though)
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