Doubt you’ll need FEA. As above, just err on the side of caution & keep an eye it; smooth radius’ & don’t worry too much about making it a bit bulky.
You could always do some simple hand calcs assuming each side of the rocker acts as a point loaded cantilever beam, but even doing this approximation, you’d need to know the kind of loading it’ll see.
(To do FEA you’d need to understand the input forces that the link would see when the bike is being ridden & without those you’ll just be pissing in the wind).
You probably need to understand the current set-up to know how you want to change it. Do you have that information? I imagine you’ll need to model the rear end of the bike & look at the kinematics of that before you can change anything…I imagine it would be pretty easy to design something that completely buggers it up, if you aren’t careful.
The easiest way to do it might be to come up with a rocker link with multiple hole mounting options from something like laser-cut folded steel that can just be welded into a box-section or whatever to make it rigid; like a test mule component. You can then use this to try out different mounting positions for the rocker link; presumably a multitude of holes for pivot, shock & rear-end of frame?
You could then try variations that you think will work & once you have those hole positions more or less right, design a ‘finished article’ from those positions & get it machined up & anodised.
It was me that did the CAD stuff for stevied’s new link. It was mainly just a case of plugging in the dims that he gave me with a bit of iteration via back & forth e-mail to make sure I’d got things how he wanted it.
Have fun with it!!