Small Engines (Lawn Mowers, etc.): replacing engine block, crankshaft diameter, bolt pattern

Question
I bought a used craftsman with a 12hp B&S motor.  Stupid me didn't checkthe oil before cranking the first time and threw I guess a rod through the block.  I have done regular maintenance onthe mower an even went as fair as replacing the valves before but is replacig the block a tough task?  I don't want to put too much more $ in it so I'm hoping to buy a used block and swirch the parts out.  Also, would it be wise to attempt to get another used engine a replace?  How would I know if the bolt pattern and size would match?  part of the model number tell you this?  Model is 281707 type 0428-01 and serial is 90120731

Answer
 Hello Wyatt:

 The Bolt Pattern isn't the Worry with a Used Engine. Match the Crankshaft Diameter and Length to yours. Never Purchase a HP Less than you have and Not More than 5 HP Higher or Damage to the Engine or Mower will Occur. Hope this Helps. I am here if you Require more Assistance. Thanks.

 Good Luck

 Respectfully

 John