I would put a 13b rotary in it. Because then i would have all the money i need to keep it running! plus save a few hundred pounds
For real
Have as many parts as possible remade in carbon fiber, remake the entire chassis in tube steel(hell, titanium while were at it), redo the whole front and rear suspension with more modern design and brakes. Motor stuff can be 2jz/APU stuff, covered before, like what the OP said. many horsepowers! 6 speed. carbon drive shaft. super tough custom diff.
Finally, The entire interior totally redone to better than new specs. good leather, rare fabrics, and better plastics!