When I built my Maplin one I'm pretty sure it was a bipolar type, probably a TIP3055, rated for 10A, good for 5A in the real world. I know that it's still lying around somewhere, when I find it I'll have a look if I can remember why I wanted to see it again. Specifications may have changed later as well. Obviously replacing like for like is the easiest way to get from a broken item to a working one, but, provided that it fits in the same place with the same pin placements, there is no reason why a more modern device should not be used, especially if it has better performance or is cheaper or more easily available.
The really important thing is to figure what killed it in the first place. Apart from overwork, the commonest thing that I ran into was the owner thinking it could be connected like a Bobs Board. Connecting the battery to the output terminals is not recommended, nor is any other random connection combination.
Do the motors just run full forward or reverse with no neutral? If so, it means that either the output transistor is shorted, internally or externally, or it is being instructed to be switched on.