The 1911 has so many after market parts available for it! I couldn't see installing that kit yourself as being too hard, provided you've opened a few guns.
have you a specific need for a left handed mag release? many just use their finger and claim it's faster. i've seen right handed shooters install ambi releases to do so. just asking.
There are no manufacturers that offer an ambi mag release on a 1911, as mentioned there are aftermarket replacements and as also mentioned, there isn't much of a need for them.
I have a Smith and Alexander style somewhere around my house and while it works OK, there isn't much of a point since using the index finger is pretty much as easy.