Zarak

Hi Everyone, So I bought an old Lexus 2007 is220d recently with its engine replaced by Lexus in 2017 and although the car has a whole year mot and everything seems to be working no check engine light etc but after driving the car for a few miles it suddenly loses power in all gears and will climb hills at 10 miles per hour. Can anyone suggest what to do? The engine is water dropping and seems very strong, like it's absolutely fine when it's cold. Oil is also fine no head gasket issue.