There are a couple of serious problems with the motor, most notably the temperatures it develops internally. There is no possible way to cool the crankshaft internally and even today's seals won't stand up to them for very long. If you go by a new RX (is it 9?) from Mazda it includes one courtesy seal replacement for as long as you own the car.

The motor is efficient. Very few moving parts and they can be moving in a hurry. High RPM are not just possible, they are practically a requirement.

Notice in that last picture (of the open instrument panel) that the tach needle is in the red (and no particular problem). The problem being weight. The motor is fairly heavy and the auxiliary systems it requires made this an 800 lb motorcycle. The whole package just overwhelmed the technology of the 70s from suspension to analog controls. Could it be made today? Very possibly, but development costs would be obscene and the mfgs balk at 'untested and unknown' items.