Welcome, and good on you for coming out of the closet. Disclaimer: I've not done this job. If the U joint is truly too big to pass through the swing arm.. (why would they do that??) removing the swing arm seems like your easiest course of action. You *could* pull the engine and transmission out and get the driveshaft out that way.
I would never consider taking a uni joint apart without having it in my hand. Even at that, it's not a job for the timid. Pressure angle had a u joint out last summer, maybe he'll chime in.
Ok, did a little research on WG. My friend Joe started a LeMans project a few years ago with great documentation. He had already pulled the swing arm before I got to it, though.. Looks like you need to do that, all right..
https://wildguzzi.com/forum/index.php?topic=68144.0