Actually it can be done, but the 1200 will run like a pig. I've heard of this job done twice and there were lots of problems. Some of the consistent problems that didn't get cured in either case were;
erratic idling, rough running at mid-high range. More importantly, chronic overheating of the heads, continually failing head gaskets because of the OH. If you really want roasted heads, buy a GL1800 that hasn't had the fix (sorry lads I couldn't resist that one :P).
The bottom line is that I wouldn't even consider touching this job, too many problems. Why not just fit a Nitro kit to the GL1200 instead, I'd say you would get some very interesting moments with that and less problems all round!