Hm, you should be veeery careful with this, as discussing "special" weapons and other war crimes violates the forum rules (AFAIK). Otherwise, it shouldn't be that hard to mod it in - a few events would probably do the job:
- An election or coup to make the US paternal autocrat (so they can declare war) and remove their peacetime penalty
- Another event to give the US temporary combat boni and/or handicap the British, triggering a delayed
- last event, that removes these effects again
Basically, it would work just like the Pearl Harbor event chain, but with different effects (maybe +100% to bombing/bombardment missions and/or reducing British GDE). Everything else is basically just polish, and can be added at leisure later. These events should probably just replace all the old US events, as most of these are highly unlikely to happen with a fascist US at war with Britain.