Stroke you can measure, just look at where you're mounting it and wiggle the bars, see what you need.
But wait...
Stopped by the dealer and he's been stewing ever since I was there last about this wiggly feeling the C5 has. He made a really good observation. The engine is a stressed member of the frame. At the front it's bolted with two heavy plates but just one large bolt that holds it to the downtube. At the rear it bolts to a subframe. That subframe bolts to the engine at the bottom and then the engine and the forward subframe at the top through two rather thing gauge plates. The swingarm bolts to the rear subframe. The top tube is a single tube affair. So single downtube, single top tube, Flexy at best. No cradle tubes under the engine since that is the engine in this case. Then the rear subframe is bolted to the front main frame, the engine, and the swingarm. Put all that felxy metal, thin gauge rear engine mounting plates, two subframes that bolt together, and all the bolt points hold the entire thing together...
No wonder it flexes!
So two ideas.
1) A reinforcing bar that goes from the top rear engine mount to the large front downtube bolt. One or two but even one should really stifffen it up.
or...
2) A reinforcing cage that goes from the two lower rear engine mounts under the engine to the large front bolt on the downtube. Could look like or be integrate into a skid plate.
Either way you're significantly stiffening the frame. idea 1 would be fairly easy to cobble together and install on the non oil fill side. Option 2 a little more involved. I think the root problem is a wiggly front end from thin fork tubes, wimpy triples, and short trail but a flexy frame like that just amplifies it once you get a little oscillation going.
Got to thank Justin for thinking of all this. Thoughts?
Oh and took a long, fast spin on the same C5 but now with almost 1000 miles instead of 700. Totally different engine! Revs more freely, shudders less when it gets to its limit, smoother, all around different and better. We started talking and I think I'll be taking this bike home tomorrow.
Scott