They didn't tweet the engine just added bigger intercooler and air intake then a map to get about 220bhp out them
Having been in touch with the original owner, he bought it new from ABT UK back in 2000/2001 when they had a UK presence.
Whilst ABT don't have specific records that go that far back they believe that it would have had a standard package with the turbo boost being raised to 1.18bar and the fuelling increased by 15% producing the 142kW at 5800rpm.
The original owner raced VWs at the time, mainly the bigger engined Beetle, but they also raced the Golfs in the 'team' and he can't remember if any further tweaks were done at the time.
The bigger intercooler was added separately to the ABT work, and I am currently waiting for the manufacturer to confirm when that was added.
I have no idea if anything else has been done, but I'll be getting it remapped once I have finished sorting the issues and found a decent remapping company.