Small Engines (Lawn Mowers, etc.): Engine, center bolt, riding mower

Question
I have a riding mower "Yard Machine", the engine blew and cracked on the outside. what would cause something like this. The machine is a 5 years old 13HP, would it be worth it to change the engine with a new one, and if so can I installed , I work in construction so I know how to handle tools!! my worry is when I get the new engine, will I be able to pull out the Stack Pulley Assembly and install it in the new one

Answer
it will cost about $300 for a new engine or you can get a short block for about half that. normally when they throw a rod, it has been run low or out of oil. the stack pulleys normally can be removed by hand after the center bolt is removed. if not slight pressure from a puller can be used. ive only had to cut 2 or 3 off out of hundreds its not normally a problem.if you need to get one, you can order them online from
http://www.buymtdonline.com