you should be able to unscrew the top of the mount from the fender, and it will drop into the fender.
a 83+ ea81 antenna will work, with a little grinding to the antenna housing.... my 82 wagon has had this done to it...
also, if you want to stick with a telescoping antenna (nice for off-roading) there's a BMW (not sure what model) that has a telescoping antenna, that's basically a direct fit, but it's about 20 times better quality. (strength and finish)