If it has a .15 in it now, there should be no problem putting a .12 in as a replacement. just be aware that you may be going from a side exhaust to a rear exhaust engine. This will require a different manifold to connect to pipe. You can go to http://www.amainhobbies.com/index.php/cPath/1_68
to get parts for your engine.
Also check to see that what type of crankshaft you have on new engine vs old engine. It may require you to get a new clutch bell to make it fit properly.