For trials and MX / Enduro use Hagon make a range of bike-specific single-spring shocks. They are steel bodied 28mm gas pressurised dampers with separator pistons, have 100mm travel, and are pre-65 eligible. They have 3-position enclosed cam preload adjusters, and run rod down. Available lengths are 330 / 340 / 350 / 360mm. Springs are black, rates 12kg to 20 kg. Although not rebuildable (the dampers are crimp-sealed around the shaft), they come with adjusting spanner and a 2-year warranty.
Use FIND MY BIKE to check which shocks are right for your bike.