I actually use an old skidplate from Moose (I think) for a KX250. Welded a plate over the old drain hole, drilled a new drain plug hole, and redrilled the back mounting hole. This thing has taken so many hits it is actually shaped around the bottom of my frame.
I have also welded on crash bars in the past, but when it came time to sell the bike, buyers were concerned the bike had been raced too hard, so made for a tough sell. (As a note, every bike I sell gets a complete paint job in green, all new plastic, good tires and chain, so looks showroom fresh.) As a result, I just add the skid plate and be done with it.
Rick