Yes, I'm pretty sure that it would be nicer to have it in the SFR space.

But just remember that the OP is more likely to carefully read your suggestions if you post text that is formulated as if written to the OP and not general text formulated for some unknown forum reader. Especially if the text seems to have as main goal to say that the OP has very limited knowledge.

A post to an unknown forum reader will sound like gloat and make the OP less likely to really spend time analyzing the text and consider any suggestions.

And don't assume that it is lack of knowledge of the processor architecture that makes them think about an extermal memory interface. I'm quite sure that quite a lot of people would think that using an external memory interface would allow the CAN controller to be connected without need to to any changes to the T51 implementation, and that this would represent the easier solution. It might be trivial to use the SFR space, but that would not be obvious to know before actually trying.