Has the fitment of the CEP kit changed the impedance of the headset? The standard David Clark H10-13.4 has an impedance of 150 Ohms (2x 300 Ohm earphones wired in parallel)
The helmets use the same impedance earphones so I can't see it being a problem.
having done a bit more research am looking for a Gentex HGU55. HSL at Shoreham advertise the name but I couldn't find this series or a price.
Flight Helmets - Flight Suits - Aviation Helmets
If you buy it from HSL you can bet they will import it for you and mark it up somewhat. The guys above quoted me approx. $55 to ship to the UK. I phoned HMRC for the commodity code (I don't have it to hand) and they said the import duty and tax would be 2.7% + the standard VAT rate. That means you could get a fibreglass HGU55 for about £600 delivered to the UK or as I said above £765 for a Kevlar HGU55 with CEP.