[Graham]

Dynamic chassis seem to vary a bit in their design. I have got 2 or 3 of the tongue section thar carries the guide which are different so it could be a Dynamic chassis. I didn't realise that Strombecker made a Mercedes body but I should think Merit is more likely.
I might be interested in the others depending on what they are. If you wanted to send some photos I can give you my email address. I have quite alot of spare parts and they can always join the queue of projects.
I have always wanted a Cox Cheetah but the only ones on Ebay are usually in the USA and way overpriced with high postage charges but one might turn up one day.

I have attched close up of the motor on the BRM
   

I am not sure how easy it is to clean the armature on these motors, but I guess if your experienced in taking them apart it maybe not an issue. I would probably be inclined to just get another one from Phil Smith. It does run the car OK, but slower.