Festiva's can be easily converted to Mazda Turbo 4 power. ...A 12-13 second Festiva? Easy.
From a collector standpoint, the Porsche 912 is an insane money maker. I've owner several in the last few years, and the money I've made off them has funded many other of my cars. They were once considered the 911's ugly little brother, but the supply of nice, unbastardized cars has dwindled. Last fall we set the world record on 912 price at auction....26,500 for a '67 912. This car would have sold for 10,000 3 years ago. Every part is available (parts aren't expensive as most think for pre-'73 911's/912's), the cars are amazingly well built, and they're so fun to drive.
Buy a nice 912, it's a solid investment.