The UCE still uses an ancient design, unlike the modern short stroke OHC design of the Himalayan and 650 twins. It's hydraulic valve lifters were fitted into the same place as the old engines mechanical lifters. It may have had a few minor tweaks, but the only "new technology" on the UCE was EFI, which I got rid of on mine.
I would agree with you as far as to say that the closest thing to a RE Iron Barrel engine available today is the RE UCE engine. I don't think they are going to last much longer either, with RE being their own biggest competitor to the UCE with their OHV 350 and already on track to release a new 350 Classic which is the biggest UCE model currently. I guess that OHV engine will become the NEXT, closest modern equivalent to a 1950's moto engine.
But, I would point out that between the IB and the UCE, the oil pump and oiling system was redesigned, and the bearings, and the piston rings revised, the timing chest redesigned, the exhaust pipe mount changed, the decompression system changed, the electric start system redesigned, the clutch redesigned, and the ignition timing technology changed. You mentioned the hydraulic lifters already, and others talked about the primary chain and the transmission. I can't think of any parts used in common, but there must be something, maybe?
It's too bad that changing one of these UCE engines from throttle body to carburetor doesn't let one toss the ECU. Perhaps somebody has figured out how to cut in an electronic ignition system that doesn't involve too much expense and rebuilding?
I don't remember the Iron Barrel having a rear disc brake? Or gas shocks on the rear? And the front fork design has been changed too from the design with the spindle on the front of the fork slider to at the bottom. The box section swinging arm is completely new as well.
To be fair, my UCE machine came from the factory with rear drum brake, Offset spindle forks, and round section swingarm.