A classic case of parallel development !
My current project is based upon a westwards extension of the Aberfeldy branch in EM and set in the 1962/64 period prior to closure in May 1965, including class 24/26/27's and the signature Thompson brake composite coach built from a Comet kit in addition to a Standard Class 4 2-6-4T.
Must get books for you are
BR Diesel Traction in Scotland
Scottish Region Colour Album No 1
Scottish Urban and Rural Branch Lines all by George O'Hara
Callender and Oban Railway (David and Charles)
However as most Class 29's were'nt converted until 1965-66 must rule those out and most class 25's would have been south of the Border at this time but 20, 21, 24, 24/1, 26, 27's should be O.K.
Some projected branches off the Callender and Oban include the Trossachs Railway and the Glen Falloch line
Please P.M. me if you require any more info/inspiration!