Carpet, trim, and interior are going to be your biggest problem. Quite simply, nobody makes them. You will have to get them off parts cars or make them yourself.
As far as the brakes go, with some work, you may be able to use the later 3.0CS brake components, so 4 wheel vented disks with 4 pots up front.
The engine should be a M10, if I am not mistaken. You could therefore do whatever else was done to a M10 to it. Converting the car to a M30 would be a major undertaking, as I do not know if it would require some cutting and welding.
Realistically, you have a pretty rare car, so there isn't a lot of upgrade information.