5.5 HP lawn mower engine seized up ! What can I do to fix the situation

It will depend on why the engine seized up. Need more information. No Oil, 2cycle or 4 cycle, something jammed around the blade not letting the engine turn? Lots of things could be the problem. Was it running and just stopped?

If it is indeed seized up, you will need to tear it down and rebuild. Is this an L head or OHV engine, vertical or horizontal shaft?. You might find an engine on E bay or Craigslist. From my past engine experience, the crank is burned, the piston may be partially melted and seized in the cylinder. Find another engine or mower. Harbor Freight sells a lot of horizontal shaft engines that get good ratings and you may be able to adapt.