There could be a couple things. Hopefully the eclip on the kick starter came off and caused no damage but I douhgt it. Sounds like it locked up when you were riding it. Worst case, Crank bit the dust. If you're already looking at porting, you should drop a stroker crank in it..... if the crank is bad.
Did you check the ring gap before putting the pistons in? Also, did you have the cylinders bored? If so, and the top end is what's locked up it could be the ring gap or to tight of a bore.
Like said above, put it in gear and push it. If the pistons can make a full revoulution, you may be in luck.