The AVL engine has issues with self-destructing sprag clutches which are expensive to replace, the UCE does not have the problem. Like them or not, a disk brake stop you quicker than a drum brake. The kick start lever on the AVL gets in the way when your foot is on the footpeg. Also, the short production run for the AVL engine suggests there may be future difficulties with spares (e.g. Rubber inlet manifold/carb manifold - call it what you like, you can't get one and it is supposed to be replaced every six thousand miles. Part No. 510821).
The top speed and mpg of AVL and UCE is much the same for both, about 80 mph after it has been run in properly, about 75 mpg on average for standard models.
I am concerned about the AVL engined thing you have seen.
2010 with drum brakes, an AVL engine, and painted white? No way, it sounds like a Bitza (Bitza this, bitza that).
I was wrong about that, it is a genuine model after all. Oops.