Firstly I needed to strip bigtrak of all unnecessary bits. The battery compartment was cleared and the bottom flap glued on, this making am ideal spot for the pi and big traks power supply to be stored adding weight to the back as the 4xd batteries did before.
I also took out the circuit board (from early 80's) and the counter sensor to work out how far bigtrak has done which you will commonly see these days on computer mice.
Next up was to add the pi, I started with a model a pi with a rtk motor controller but figured I couldn't do much more than control the motors (newer versions have longer gpio pins which give you access to the gpio connections) so eventually swapped the MicRoCon from 4tronixs and started up the pi which then needed scratch gpio added.
Once up and running I placed the covers back on without screwing them and hey presto a faster tougher (and noisier) bigtrak. I have a laser module to go in it at a later date and have already wired the speaker to the pi just need to come up with some sound files for it.