Trumpeter do the US Navy LCAC in 1/144 and 1/72 scales, and they're excellent kits too, but toys in comparison with an SRN4!
I've been working on making the Airfix kit into a Super 4 for some while, but the standard kit is a Mk1, and needs converting to a Mk2 before you even start the stretch to the longer version. As the kit is really vintage it has HUNDREDS of parts, made at the time when the parts count indicated the value of the kit perhaps, and there's quite a lot of work involved in doing my conversion, but I suspect your idea would be easier.
You don't have to bother about scale-like appearance of the forward cabins etc. The 'craft would need some notional reinforcement to carry a chopper on the roof, it's not that strong up there and you're only allowed to walk on the railed-in walkways, all the rest is a 'No Step' area. The propulsion units would need moving too, as well as shifting the cushion intakes, as the chopper pad would take up most of the roof.
But it sounds a great idea, BHC would have loved to have done it I'm sure.
In passing, the LCAC has almost the same footprint as a BH7 Wellington 'craft, so I'm looking at doing one of them.....