Sorry to hear of your stock problem, but Marlin is good about things like that and you may find that your replacement stock is a pretty nice piece of wood. As for the sight problem, shooting low with factory sights and extra power ammo in a .45-70 is to be expected. The rifle is regulated for the leisurely pace of the factory load, which allows lots of time for the piece to recoil as the bullet meanders up the bore. Low velocity loads always shoot higher than zingers. A good receiver rear/post front will solve your sighting problems: the Ashley brand that others have mentioned are good, but pricey. I put a Williams Foolproof rear and old Redfield Sourdough front post on my own M1895S. Puh-leeeze don't think of putting a 'scope on it, like the gun rag writers want you to do!!!!!
If they take our guns, I intend to let my hair grow long and acquire the jawbone of an ass.