Quite apart from the 'delicacy' of the exterior mechanism parts of the O2, there's the 'interesting' constructional scheme to adapt an A1 boiler and cab unit for a fit. The boiler interior is filled near solid with a pair of castings with the motor sandwiched between, which the plastic body shell then closely covers. Not an 'adaption friendly' construction when looking to make a conversion. As above, refer to the O2 thread for much more information.
That apart, the visible powered mechanism and footplate - if the undersize driving wheels are acceptable - would do for a P1 lookee-likee, and the flush sided tender body is robust enough to take new upper sides, but will need replacement frames.
Before walking that road I am going to wait to see if Heljan will offer the GNR cab and tender version. Because if not, that's the first project on the basis of one of these...