Thanks Ellis, what I was thinking was of reducing the motor shaft diameter and press the input gear on to it, then mount it directly to the gearbox just like the original motor (maybe with some minor machining) But if the banebots are of similar size but with more power then I'll have a look at their specs to compare... I think it will pay off to invest a bit more in a better matched motor rather than end up burning up motors not suited for the job.
Something else, don't know if it has any effect on what happened, the gearboxes I run are 1:36 ratio...