I spoke to Mark Petch about it, he said that they had so many spares when they bought the car from Walter Wolf, it was easy to do. I don't think the car lacked for anything mechanically, it appears to have all of the RS500 updated parts such as the huge mag trailing arms with relocated mounting points, brakes, suspension, etc. So in theory at least, it should have been as good as the other car. The rest would come down to how well it was set up,how well it was driven, and how high the boost was wound up, such was the way of these old cars
The values of Gp.A RS500's have skyrocketed in recent years, and the XR4Ti was a way for my friend to get into a proper Gp.A car for sensible money. There is also the fun of discovering its history,"finding" missing parts and the rebuild itself, if you enjoy that sort of thing. (and we do!)