I've been asked quite a few questions about this project and I can now exclusively reveal that the author of the project is Leo White. Not only a whiz with RISC OS, but also has quite the talent with Electronics. Now if the following doesn't scare you, then I am sure you will be able to make your own Big Traks in no-time, but seriously this is NOT for the faint hearted. Remember Leo also enjoys hacking operating systems in his spare time...So here is what he did.
He stripped out the motor unit from the Big Trak, removing the existing controller board and then he connected a replacement motor driver chip. He then hard coded the circuit to control the motors, connected his Raspberry Pi, got that to control it and added an extra mobile phone type battery to power the Pi and extra electronics.
When he came to Cambridge, he of course tidied it all up... After all you can't be shabby when coming to the Uni! And he told me that in the last couple of days he's been working on the rocket launcher!
You can keep an eye on his progress at his Google+ page: https://plus.google.com/107287498294316622336/posts and he says that he'll be putting instructions together at some point, but we'd better not disturb this genius at work.