Engine:

• Inexpensive Pressure-Relief Mod
  Install this mod at your own risk!
  It is purported to minimize wet sumping caused by sustained high RPM operation of modified Evolution Engines.
  See more on this discussion here which originally began with an issue regarding 04 and up oil usage:
  http://xlforum.net/forums/showthread.php?p=754838#post754838
• Custom Oil Pump Hoses
• Adding a Vent Between the Oil Tank and the Cam Chest
• Installing the Ultimate Crankcase Vent
• 86-90 Oil Filter Mount Mods
• 07 Up Oil Pump Mod For Use in 86-90 Engines
• 86-90 Engine Breather Mod Using #4 Cam as an Oil Slinger
• Windage Tray / Flywheel Scraper

Primary:

• Homemade Primary Oil Sight Gauge

Generally speaking, all modifications to the factory engine breathing system should be done at your own risk. ¹⁾
The liability concern is due to somebody installing something wrong or if they lost oil pressure (or suffered engine damage).
Companies don't want to take that risk concerning mods.
That's why most won't sell 'kits' for most of these mods.
Modding factory components is usually done entirely at the users discretion.