From what I've read
single to twin =
twin scroll sump, oil pick up, dipstick and tubing ,twin scroll equal length headers ,twin up pipe ,pipe work for turbo ie oil and coolant, twin scroll down pipe.
So that should mean twin to single you want to get
sump, oil pick up, dipstick and tubing, unequal lengths headers, up pipe, down pipe, oil and water lines to turbo. A turbo and remap.
Ive got a number for a decent breaker if you want it to see what bits he can get you, he is very knowledgeable so would be able confirm exactly what you need.