I have both an XB (Buttercup!) and a current Sporty. The principle difference between the engines is that the XB has down-draft heads and a centrally mounted fuel injector and air filter. The thing that looks like a gas tank is only the air filter cover. So the route for air into the air inlet is dead centre above the heads, in line with the Sporty's top frame tube.

I can imagine that it would be pretty straight-forward to replace the top tube with a twin tube arrangement, even to adapt a stock gas tank to house the Buell air filter, but that makes it a challenge to find a way to carry some gas! On an XB that is in the alloy frame.

 

You might want to look at the XR1200. HD put the downdraft XB engine in it somehow. Maybe in the parts book at a dealer would show how. The lower engine cases on the
Buell are a little different also. HD married the XL case to the XB upper end to mount into an XL frame.

I have an 08 Buell Ulysses XB, 09 XR1200 and a XL1200. Now I am curious to see under the XR tank set up and see how they did it. Looks like I will be spending time in the garage tomorrow.

 

We can study the XR on line here, the Buells are not so easy on-line. As already mentioned XB heads are down-draft, XR are side-draft, like the other Sporties.

 

Did everyone miss the link above to the guy in Poland that did it?