What a nice project .. the search lead me to your post .
last week I had an offer to buy a 1999 911 GT3 with bad engine for about 6K $!!
I was thinking to put 1UZ or L98 engine on it.
show us more pictures for the water cooling system and exhaust system
This car is 100% built in Denmark ...
Performance (kW/hp/min) 810 / 1104 / 6900
Max. torque (Nm/min) 1430 / 4500
Top speed (km/h) 375 (electronically controlled)
Acceleration 0–100 km/h (s) 3,0
Hp/weight 802 bhp per tonne
What I remember is 650 RPM +- 50 for idle speed. you could play with TPS till you reach acceptable reading.
Or, Reset the ECU and fix the TPS in the middle ... use it for one day ... if its not fixed try play with ISC.