Making procedural terrain match an existing shape is very difficult. An approach that we've used on a few different projects now is to model the terrain in a 3D package, export some heightfields, bring them into Terragen, add procedural details and render in Terragen. If you already have a model of the mountain, that's even better.
Another thing. You can make your renders faster by using planes that are the same size as your heightfields, instead of trying to render the whole planet. A completely flat horizon is slow to render at high resolution, even when it gets covered up by foreground mountains. Having the whole planet is convenient, but if you're willing to spend the time to build pieces of terrain everywhere they need to be, and only render those pieces, it might render a lot faster.