1998 Daewoo Leganza no final drive

i have had a recon engine put into the car and its great but when i put it in gear nothing happens. i lent the car to my son in law and he seized it up but i don't know if he had it towed back to his house, i had it transported to my house where the repairs were carried out, it seems the oil pump packed in and he literally filled the engine full of oil. the mechanic says there is nothing wrong with the gearbox. please help