Thank you to both HOSO&NZ and CJ Meyers.
Drilling a clearance hole through the opposite side plate to allow access is not feasible on this particular motor. The field laminations are in the way.
The post made by CJ Meyers is very detailed, and contains a lot of good information.
Please extend my thanks to your train show friend too.
I was hoping to avoid having to seperate the motor sides, but it looks like its the only way to get the job done properly.