Swapping motor on Plarail Nagasaki 2x series 3000

So I finally found one Plarail Nagasaki 2x series 3000 on ebay. Sadly the original motor is fried due to battery corrosion. I know people will get upset but I replaced the damage battery terminal with Peter Sam's. Works like a dream. This train has so many pieces beneath the chassis so be careful before you put the chassis apart. I am trying to figure how not to make the train sag.