There are quite a few differences, it is far more than the ECU. However to get a 75 to 90 is easy, remap by Stealth or whoever! The 75 also has rear drum brakes instead of discs otherwise most of teh difference is with the engine. Oh, and a slightly lower top gear ratio. If I had a '90' it would have to be a phenominal deal to pursuade me to go to a 75, especially when a remapped 90 will get to about 120 horses...