If the crank bearings are really sad the main bearing can be replaced but you need specialised gear to push it apart and put it nack together cleanly and straight a really good idea to have a engine reconditioner to do that (Darryl and I were talking about that tonight actually!) they are very hard to get aligned straight.
The main crank bearings are a slide on and off affair, sometimes require a bit of encouragement using a suitable drift carefully to put them on, being careful only to push on the inner part of the bearing.
