I take it, that the "motor" is not blowing blue smoke out the exhaust, or is using oil, or broke a spring, or has a flat lobe on the cam, or? If not and you're reasonably sure it's a lifter and it's bad...really bad and you tried all the other "tricks" (i.e.: lifter supplements from an auto parts store)...Then why not pull the valve covers, back the rocker arms off, pull the push rods, pull the intake and replace the lifters (all of them)? Be sure to lube the new lifters, put it all back together and let her rip. It's not a "big job" or really that big of a deal! Of course this is all sight un-seen, but don't try to make lemonade from lemons if it's not hot outside. Good Luck!