This is not a true conversion as the original chassis is from Trix Twin tank loco. I received it without any collectors and it took a bit of working out. I discovered that Trix did not use the wheels as collector but had seperate collectors behind each wheel running on the track. Even though the wheels are insulated from each other, and the chassis this explained the lack of any wheeled based collector fittings.
One terminal of the motor was connected directly to the chassis so fiiting a simple collector was straight forward. Determining the thread in the centre was not. It is around 2.5mm. It is not M3 or  6BA or 8BA. As yet I have not worked out what it is. Fortunately a screw from a set of Trix points fits. At least Trix were consistent on fasters across their product range.
An old model, judging by the motor, so sufficient mass to ensure sufficient downlard force.
As yet I have not fitted proper wheel collectors. The jury rigged wire works quite well. Especially after the wheel was cleaned up.