I've been kicking the idea of doing a 912 "hot rod" so I'd rebuild the VW, or have one built (maybe Pat at CB Performance), up grade the suspension and dial it in, etc... A 912 will always be a 912. They have a smaller market and they engine parts will continue to get more and more expensive and less accessible where as VW parts have a huge aftermarket source. The original motor is long gone so no matter what, unless you have a stroke of luck, you'll never have a #s matching car. Yes, I know, the enthusiasts will poo poo the idea but it is what it is.
You already have the beginnings of a great little hot rod, finish it and enjoy driving it. Besides, they say those older cars are better with a light weight motor in the back
There's great link somewhere of a guy in france who restored an old 912 with a VW in it. It's really cool. He's kind of a "Magnus Walker Junior" of the long hood 912s