[WFBowen]

Upgrading a 912, as Darryl mentioned, can get real pricey for what you get. One of the advantages of the 912 motor is that it already represents the final development of the 356 motors, which themselves were major upgrades on the VW engine platform. That said, my advice is to take your stock motor and (if it isn't already there) get it absolutely up to specs - carbs/ignition/piston-cyl - everything rebuilt. Anything beyond a stock 912 also brings it's share of baggage - the Porsche engineers probably knew how to get 250bhp from their 4-banger, but also knew at what point to stop.
As Chas mentioned, you could drop in a 911 motor, but then I know several 6 cyl owners who always want to move up "a letter" (i.e. T to E to S to RS 2.7 to Turbo & on up the ladder.) Also, you might check into a Type IV conversion, but , by the time you get the car off it's jackstands, it's not that inexpensive.
I've had maybe 75 sportscars of all kinds over the decades & probably my favorite was the 356A Super. Those of us who came from the 356 philosophy (read: old guys) seem to recognize the 912 as an extension of that car - it's not so much a speed/power thing as it is an efficiency/engineering package that works better that just about any car ever built. A really tight, sweet-running 912 is a fantastic driving car in its own right (even though any college co-ed in her new Civic can beat it - but that's why Porsche makes twin-turbos.)
JMO,
Bill.