Keep it a 2.7 if the P&C in in spec. No replacement for displacement. As mentioned above if not already done open up the ports and then its all ablout the Cams you pick. You can use E, S mod solex (DC30) or Mod S (DC40) cams. A friend of mine built a 2.7 with RS pistons and Mod S cams. I had this installed in my 914-6 for a while as I rebuilt my motor. This was a great combination. He has since sold the engine, specs and dyno in link below.
http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/1074394-911-engine-sale.html
Previously this engine had E cams and while fun it was not as fun as with the Mod S cams. FYI the curve for distributor in most of the 2.4 and 2.7 are the same.
john