hi martin,
i cannot comment on Sweet Violet or Sweet Pea, but i have quite a bit of experience of dealing with Don Young's Railmotor design which as you state was described in ME 1968/69, though under the title 'push pull locomotive' or '0-4-0 locomotive'.
Railmotor No.3, the LSWR S14 variant, was described by Steve Titley and Harry Lumb in Don's magazine LLAS in a single article, with a sheet of drawing modifications by Don. it doesnt deal with all the alterations required.
i have all the articles, but a bit much to copy them all, but happy to help if you have any missing etc.
cheers,
julian