8 mph motors

[racingrevell]

Hi I bought my son a pre-built toylander and while it has two motors at 4mph it is painfully slow, like watch paint dry to my 7 year old.  I have seen some 8mph motors and yes I know the laws for pavement speed, but we have our own private land.  Does anyone know of a direct fit replacement for the standard 4mph motors supplied by Richard?