well i would have suggested a rubber hammer but youve already tried it! if it still doesnt come off with some perserverence try putting the cylinder head back on and turn the bike over. the compression might shift it.
few years back id seized my yz250 absolutely solid and had the same problem. i ended up putting a strop round the head and attatching some chainblocks
hope you dont have to go to that extent
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wadda ya mean its too fast!