avionic type, I was working and licensed (mainly engines) on the merchantman right up to the retirement of “EP”. Our scheduled boroscope inspections of the compressor section were carried out at the interstage casing on left hand side of the engine. This involved removing the triangular plate secured with qty 3 9/16 bolts, then with a rigid 90 degree scope could easily view the LP 5 and HP 1 stages turning the prop by hand had sufficient friction to rotate the HP as well. The condition of the compressor at this point was considered to give a representative picture of the state of the overall compressor. On the side of each nacelle is a quick access panel when opened you will see a small round panel with a lever opening this allows for a 4th stage turbine inspection using a torch and mirror.
Further to your starter motor problem, the Rolls Royce museum in Derby has Tyne 506 on display. We helped build it up for them and it did have a starter fitted then, possible they would do a swap with you.
Regards.