There are no plastic parts on the original Mamod RW2 Lumber Wagon (the logs are polystyrene).
Best to start by removing the drive bands then the three logs.
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The chassis side frames are held in pockets of the back of the drag beams by the buffers. Remove the buffers, then ease out the chassis side frame.
NOTE - the black baseplate is riveted to the drag beams & chassis side frames so do not force out the side frames too much.
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With both chassis side frames free of the drag beam you should be able to pop out the wheels. Fit Gauge 1 wheels, and reverse disassembly process.
Chris Cairns