I wouldn't bother with the screemer mate the external wastegate sounds much better on it's own,besides that the law might pull you over more with a screemer!
You need to remain as inconspicuos as possible these days, too many moaners about loud cars at the moment especially loud exhausts a screemer is going to attract unecessary attention
Would of thought you could change the exhaust housing on the turbo quit easy, but then cost of external wastegate/manifold/ screamer etc !!
I did love the screamer pipe on mine but after a while it gets too much!!
weld the flapper up and stick a wastegate flange on the standard manifold..(you'd need to modify inside the manifold for it to work well due to that divider inbetween the runners), alternatively buy a manifold with a wastegate flange on it already.
personally wouldn't bother though, seems like a waste of time.
No matter what, it requires taking the turbo off and either changing the turbine housing or welding the wastegate shut?
Sounds to me like the easiest solution is to take the whole lot of the engine and replace it with one from somewhere else (then sell the current setup to someone who's happy with an internal wastegate). - I wasn't too far off this time, I must be learning something after all.