I think getting more pop out of a 912 is a bit of a quandry. I know of guys who increased performance with a big bore kit, rebuilt carbs and updated dist. It depends on how fast you want to go, go real fast... get a 911. At best count on spending several grand even if you do the work yourself.
Parts are available here at Pelican, ************* and classicandspeedparts.com/
Ask this question over at
http://www.912bbs.org/ they have more action on the site.
Chas.
356
912