Depends on what seized. If it's the piston, you'll need to bore the cylinder and put a top end kit in it (piston, wrist pin, needle bearing, gaskets). If it's the bottom rod bearing (which it probably is), you'll need to tear the engine down, split the case and remove the crank, have a new rod and bearing installed, replace the crank bearings and seals, and put a top end in it if it needs it--or rings atleast. It may have seized due to lack of lubrication (not enough oil in premix, or oil injection system failed if it has one), not warming it up enough (if the piston's siezed, this is a common cause--cold seized), or it may have just been worn out.
'00 416ex -- 11:1 Wiseco, stage 1 HotCams, LEXX slip-on w/ modded header, ASR +2 arms, 450r shocks, G-Force axle, FST style cooler, Trail Tech bars and clamp
Temporarily retired from my extremely short (but successful) racing career.
Maybe some IXCR in the near future.
A government big enough to give you everything you want is strong enough to take everything you have. - Thomas Jefferson
None are more hopelessly enslaved than those who falsely believe they are free.
-- Johann Wolfgang von Goethe