What's your defination of "blown"? I have rebuilt many engines and the 250EX is plentiful as far as used parts. I would venture to guess it could be rebuilt for $450 to $850 depending on the amount of damage. I have had motors with bad cranks or broken engine cases shipped to me, rebuilt them and shipped them back for less than $1000.
Here's the scoop: You should be able to use an engine from a 2003 to 2008 TRX250EX, a 2002 to 2011 TRX250TE or TRX250TM, and 2009 to 1011 TRX250X. At least the crankshafts, engine cases, and cylinder heads are the same on these models for the years listed. It appears there was no 2010 model for any of these. This info might help make your search easier.
If you are interested, I found 2 complete engines. a 2006 for $750 and a 2007 for $800 + $100 shipping from Kentucky. Or if you are interested in rebuilding, a complete lower end for $475 free shipping from Missouri, and a set of cases for $175 free shipping from Penn., but you would have to get a new crankshaft from Honda for about $270.00
Using used parts, the motor could probably be rebuilt for $850 with the only guarantee being against defective parts. Keep in mind that these motors are push-rod valve train motors. They are considered utility engines NOT perfromance engines. I'm guessing your motor exploded due to high RPM use. If you are trying to run this machine on a track, I would recommend salvaging it and getting a sport model that is more suited to the way you are using it.
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