156 Engine Fire

Need some serious help!!!
I drive a 2001 Alfa Romeo 156 Sportswagon. Recently the car started revving high while in neutral and a new Throttle Potensiometer was fitted. However the problem didn’t go away and the engine still revved high every now and again. Last week another throttle sensor was fitted (a sonic? or phonic? wheel sensor). The following day, while driving the car, it lost power and a noise could be heard from the engine if I tried to accelerate (the noise was almost like the exhaust was going). I eased off the accelerator to lessen the noise and then saw an engine warning light flash on an off. I slowed the car and as I came to a stop I realised the engine was actually on fire.
The fire damage to the engine was confined to the top of the engine and to the left side as you look at the engine. It was also confined to the part of the engine that is closest to the windscreen.
Wiring is fine coming from battery. Battery is fine. Dashboard lights still work.
Anybody got any ideas at all about what could have been the cause of an engine fire like this?? Help, would be very much appreciated.