Hey Rick!
I'm in the same place as you, considering what I want to do to my 912 project's engine. From what I've researched, there's not much you can do to increase performance without splitting the case ($$$ ka-ching $$$). I'm considering a SCAT light-weight crank, big-bore kit, mild cam and having my Solex 40 PII-4 carbs professionally rebuilt.
If you are just getting started, I can recommend the best book on the subject:
http://www.912registry.org/sponsors/complete912.htm The best $25 you'll spend on your 912 project.