Yes, you can replace a couple of lifters pretty safely. Just do the 20 minute run-in at fast idle like you would do with a new cam/lifter installation. Stovebolt lifters seem to have a chronic problem with being unable to purge air out of the lifters, so you might also try putting a shallow groove in the ball-shaped bottom of the pushrod with a Dremel tool. I think there's a tech tip on that somewhere on the website. It's also possible to remove the lifters that are giving problems, take them apart, and do a thorough cleanup with spray carb cleaner and reinstall them. Just don't try to intermix parts- - - -dismantle and clean one lifter at a time.
Jerry
