It's been a while since I worked in N scale so I had to think about this! The clayliner was a mixed pool of standard BR 5 plank 10' wheelbase wagons which are readily available from I think all 3 manufacturers, and highfits which are available as a kit from the N Gauge Society. Alternatively it's a pretty simple kitbash to cut the wagon ends out of a Farish 5 plank and insert some corrugated styrene. The N Gauge Society does also have the metal bodied version if you'd like to add a bit of variety to a rake. Subtle differences between brake gear types are simply too small to appreciate in 2mm scale so I never even bothered - once weathered they're such a close fit that nobody notices anyway.
Use masking tape for your sheet, with a simulated load underneath to create the "bulge" - sheeted opens were always loaded above the wagonsides to create a "hill" so that rainwater wouldn't pool on the sheet. The sheets were various shades of grey, black, dark blue, etc. but if you're feeling adventurous, use some clear tape for a couple of the sheets, and give a light mist of white weathering with an airbrush - there were some experimental clear plastic sheets for these wagons and that adds a little interest.
For the roller bearings, I just used a sharp knife to remove the old friction bearing, and replaced it with a piece cut off the end of a styrene rod of appropriate diameter. This is pretty crude but to be honest with N scale it's so small that this actually does the trick at normal viewing distances.