I am sure that you have aerated the soil and put a sand peat moss mixture down. Sometimes you have to go 3-5 inches deep. I would not recommend going with a aerator that you can rent, check to see about getting it done professionally. that way they have stronger tynes and larger holes.
if you know anyone in the auto or equipment repair, check with them to see if you could put it in their oil fired heater. Plus see if they still scrap out filters. We use to cut them open and recycle the guts and scrap out the cases
for a mower that age I would run 20w50 motor oil. I run that motor oil in all of the engines that are in my fleet of greens mowers, bunker rakes and out front rotary mowers. One thing that you could do is when you finish mowing, take the engine down and install a set of chrome rings and replace...
I have had more trouble with those electric gas shut off solenoids. I have taken those off and plugged the hole. Plus I have removed the filters that you find in the bottom of the fuel tanks and put inline fuel filters in the system. What most people need to do before they buy a new carb is to...