Well, first you want to have even compression (within 10%) between all four cylinders. That is more important than having 125-140 psi. And, don't believe everything you read on the internet.
Second, you have not told us whether or not the tractor has flat-top pistons or stepped pistons, or power crater pistons. I have a rebuilt Super M with overbore (4-1/8") stepped pistons (around 1/4" step) and it registers 125+/- per cylinder with an 8060 head. If it had the stock flat-tops, I'd guess it would be around 90 to 95. It runs very well, by the way - and many tractors do with 85-95 psi while making a ample power.
You sure as heck don't need 93 octane with an 85-90 psi compression.
After 10 years of not running good, I'd look into worn cam, worn distributor, etc., before I worried about 90lb compression.
That is another thing..... did you swap distributors? Various models have different advance curves and different maximum advance degrees.
Let us know what cures your problem when you find out. We all learn from each other.
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