I used Army Green Matte PLA from Amolen. The real locomotives may well have been a darker or greener shade. If you want to paint the finished model you will find Tamiya Acrylic paints work well on this plastic. This plastic responds well to either Superglue or Plastic Weld solvent Working parts are held together with 10BA screws to permit future disassembly for repair/replacement. Holes for the screws need to be tapped 10BA Use 2mm silver steel for axles. Motor and gears came fromTechnobotsonline.com. Model Motor size B (1400-021), MOD 0.5 Plastic Worm Gear 10mm (4600-039), MOD 0.5 Plastic Gear 30T (4600-013) Note axle holes in frames and gearbox will need to be cleaned up, I used a 2.1mm drill 3d printed wheels will need axle holes cleaned up; I used a 2mm drill which left them a tight fit. If you find them loose on axles then secure with superglue. Serv-o-Link delrin chain from Motionco, You need 6 1/2" (that is 52 links) It would be best to use 16 tooth delrin sprockets from Motionco. These were out of stock so I printed my own.
Don't look too closely at the pictures. You may see that I did not have stringing under control when printing some parts.
The following gallery of pictures shows the assembly sequence that worked for me.
SIMPLEX 20HP information sources
Motor Rail Catalogues Alan M Keef Narrow Gauge & Industrial Railway Modelling Review Issue No’s 38, 39, 58 Simplex Internal Combustion Locomotives Moseley railway Trust WDLR Album Roy C Link The Early Years Of the Motor Rail & Tram Car Company W J K Davies Motor Rail Ltd Alan M Keef