Let's get started with the swingarm.
The swingarm is made from three pieces. There is the cast front section that houses the bearings and there are two forged arms. They are connected with four square-drive bolts per side. In addition to the bolts, the arms are also epoxied to the front piece. The epoxy looks just like JBweld.
Here's what I think the main problem is:
The arms are C-sections where most swingarms are fully boxed. I will cut pieces of aluminum to fit the insides of the swingarm and weld them in to make a fully boxed arm.
Another aspect of the swingarm that might be causing flex is this E-section part on the outside:
I will also box this in. I'm guessing after the whole thing is boxed, this E-section center will be unnecessary so I plan to drill out a lot of it before I box it in. I hope to be able to improve the swingarm strength without adding weight.
I also plan to weld the arms to the front section and remove the bolts. I don't know what alloys the swingarm is made out of so this leaves a lot of guess work. There is a lot of room for disaster with this project. It looks like a used swingarm can be had off eBay for about $100 so I'm not risking too much here.
I have chosen 5052 Alu for boxing the rear swingarm. 5052 provides the best as-welded (no temper) strength. ESAB recommends 5356 filler rod for the least chance of cracking when welding 5052, so that's what I'm going with. The biggest concern I have is whether the front piece and arms will both accept the 5052 filler so I can weld the thing together. The next biggest concern is if the epoxy will contaminate the welds.
The aluminum and filler rod were not available locally so I ordered them from McMaster-Carr. I got the aluminum a few days ago and the filler rod arrived today so I should be able to get started on this next week.