Hi - will be racing at Anglesey with CSCC in a few weeks - come and see us!
There are a number of firms who will custom-build a cage for you, but of course at a cost....eg
https://www.customcages.co.uk/bespoke-roll-cage-service , and of course Safety Devices, who have just built one for a friend's Alfa 4C....
I would have thought that a cage meeting US requirements would be OK, but it would be wise to check with Motorsport UK before shopping across the Atlantic