The only nag-box I get is a requirement for a number code if I post photos.

The SU rig is an MGB setup, though for earlier year cars in the US market. The set you see was lifted from a rotbox '72 MGB.

In the cars destined for the North American market, later ones like mine got a truly horrid Zenith Stromberg rig set up to meet early US emission regulations, rules the SU-equipped cars couldn't meet apparently. The ZS carb is not at all my favourite for a number of reasons, the inlet looked like something a plumber would cobble together out of urinal plumbing, and the added "features" strangled the engine horribly.

The Weber that was on the car was an owner replacement, in itself a vast improvement. The HS4 SU pair are a little stronger yet; though perhaps not as fiddle-free as the Weber was, they still are lightyears more reliable than the original single-Stromberg.

In the US, each state makes its own regulations as to registration, licensing, taxing, and inspection, so yes there is the equivalent of an MoT inspection requirement and of the functions of DVLA but not at the national level. There are variations between the states as to inspection requirements and frequencies, taxing rates, &c