To check if the engines are in anyway similar,you need to check the engine numbers of each.The numbers should be stamped on the front of the engine block.
The numbers for the Megane should either be E7J 624 or E7J 764.
The numbers on the 19 could be one of many,but if it starts with the E7J part,you could be in luck.The block and head should be the same,but the ancillaries(manifolds,alternator etc) will need to swapped over from the Megane.
If the engine is seized in the Megane,then theres no point in repairing it as the cost would be prohibitive.If the 19 engine doesn't fit,then one from a breakers would be £250 to £350 depending on mileage and amount of warranty given.
The best way to check to see if an engine is seized is to try and turn the crank shaft pulley bolt clockwise with a spanner(with the car out of gear).Seized engines are very uncommon,but the cambelt could have snapped on your Megane giving the impression that it is seized,but even if it is just the belt gone,then repair is still expensive....
Good luck,and let us know how it goes.