The standard engine is a 6.5L turbodiesel. For a few years, a 350 (cubic inch) gasoline engine was available, but it was discontinued as of 1997. 6.2L and 6.5L normally-aspirated (non-turbo) diesel engines were available in earlier models. The newer Alpha version was equipped with a Duramax diesel engine.
Depending on the year I would replace the motor for a newer version of the same motor.