I would bet that the bad lifter has worn out the lob in the cam. not saying that one can't collapse and never come back up. but I would go for a flat lobe or a bent push rod. usually a lifter will come and go. but if you get the man to change the seal be sure and ask him to check the main bearings and end play in the crank. if they are good then think about the lifter problem. you can get a lot of use out of a not quite right engine if your not doing any long halls. but you cant make one heal it's self. you have to live with the problems it has. the old hub cap slid under the dust cover would take care of the mess. till you can do it proper. you are right one car at a time is all you can do. let the other one wait till you can get to it.