Nice load of information there to get some advice.
You need to test for compression by taking the plug out and pop finger over hole when trying to start. Give away is how hard is it to kick over. Is there much pressure on the kick start, does it gind away?. You can get it tested.
However a bearing may have popped in the engine at the bottom of the crank. The extra pressure from the bigger top end has made it strain!
By what you are saying its defo had a seizure you just need to find out where!
bb
May 2010