Steve, may be a long shot but have you tried Custom Chrome in Nuneaton?
They are a relatively small custom exhaust manufacturer and fitter, so may be happier to work with smaller numbers.
The silencers they fitted for me 6-7 years ago are quite tame compared to x-powers as I didn?t what anything too loud, but they have no resonance and great at motorway cruising speeds.
I was pretty impressed with the quality for the cost, fully fitted with a choice of tailpipes, the cost was nearly half what used x-powers were selling for at the time.
Back then the owner of CC was driving a GT500 so had a good idea on what would work on a 260.
May be worth a chat to see if they could build x-power replicas, though they may need to see & hear some to get an idea of what your after.