250-500cc please help with a water pump impeller

i have a cagiva wmx 250 and i beak the engine....i buy a "new" engine from husqvarna and the man that sold me the engine told me that's ok for my bike it's the same engine...but is not the same my rezonator does not fit it's smaller and the engine has that exit 60 mm + i need a water pump impeller can you help me with an answer ...excuse my enghlish but i don't write so well