easy job, drill a pilot hole in the old sight glass (it's plastic) and partly screw in an appropriately sized screw, which can then be grabbed with a pair of pliers to yank out the old sight glass. Clean the hole thoroughly and tap in a new one. It is a simple friction fit. On oilheads they sometimes pop out with catastrophic consequences because of the greater crankcase pressure when both pistons are at the bottom of their stroke. Not a problem with K's since the pistons don't all move up and down in sync. Some oilhead owners have rigged mechanical fasteners to keep the sightglass put.
