And time to perhaps give this discussion a little bump.
When I had the engine apart on 1525, I looked at it with an eye for how viable a larger displacement, counterbalanced version could be, within the general architecture of the engine that exists. That suggests something that would fit in the same frame and look similar, if not identical.
It looks like there is real estate in the lower, front crankcase, where a counterbalancer could be designed in. Because of the cylinder bore sizes and their relationship to the cases, cams and timing drive, the easiest way to more displacement would be more stroke. I don't think you could get to 110 cubic inches but, 100 should be do-able in those cases. Going to more bore would require moving the cam drive further outboard, along with a number of other components that would have to follow.
It's also a different world with respect to emissions and the X ran hot. You'd almost guarantee that you'd need to go water cooling this time. Maybe it would be water-assisted cooling like the Milwaukee Eight, or there could be a form-fitting radiator stashed between the frame down tubes to take care of full water jackets. I think the exterior look of the engine could be done, with integral water jackets and cosmetic fins, to look like what we already have.
Overall, the bike had a unique look and that could be maintained with a freshened-up engineering package underneath it.
Today I was riding down the highway on 333 and a guy on a brand new Road Glide started to pass a multi-lane area. He came up hard but, then throttled back just as he came up on my side. It was either the pause of knowing what the X was and having never seen one, or not knowing what it was in the first place.
When he looked over and caught my eye, I got an odd wave as I saw him look down at the front end. Only after completing his pass did I see the paper dealer placard still on the back of his bike.
