You've asked this same question before, and it has been explained a few times, and unfortunately the answer hasn't changed! It's still not practical, and while something like a SBC V8 will give you more power, it will also change the center of gravity on the 912 to such a degree that you will be unable to take ANY turn at more than 10-15 MPH (which will be extremely disappointing when those stock 912s go through the same turn at 60mph+).

But you're not really looking for reason... so here you go:

Strip the 912 down to a bare shell... easy if you do Vag 15.7 this yourself, a few thousand dollars if you have a pro do it.

Redo the body to include factory turbo flares and the RS style front (for wider rubber and an external oil cooler). Replace the engine lid with a carbon fiber or fiberglass model that has either a duck tail or carrera tail built in (trim weight off the back).

Order some massive torsion bars, all plastic bushings, Bilstein sports for each corner, and all the Smart Racing or TRG goodies you need (Spring plates, front monoballs, and adjustable ARBs).

Add 930 brakes, rotors, and Master Cylinder... and since you're there, add a bias adjustment since the stock adjustment doesn't have enough brakes to the back.

Pick up some lightweight BBS race wheels, circa 1980 or so... 7x16" front, and 10x16" for the back... plan to get some Kuhmo V710s or Hoosiers for them.

Replace the interior with RS trim, and pick up a pair GM Tech 2 of Recaro race shells. Probably a good idea to go ahead and add 5 or 6 point harnesses and roll bar or roll cage at this point along with back bracing (to complete the safety aspects the lighter and more supportive race shells will bring with them).

Transplant a 915 transmission with the factory LSD and add the WEVO linkage, gates, and shifter.