Oil blow-by in my air compressor

I have a Sears 2HP/20Gal mobile air compressor that I bought new about 25
years ago when I was a teen. Note that back then, Sears compressors were not
sold with over-rated HP claims: mine has a 2HP/15Amp USA-made, Century Motor
by Gould Inc.
Even though I've always kept it topped up with air compressor oil, it has
succumbed to wear and tear (I admit, I've demanded much more air volume from
it that it's capable of delivering, particularly when sandblasting). It now
blows a small amount of oil into the tank. Not enough to significantly
diminish the oil level in the crankcase, but enough that a milky yellow
oil-water emulsion is blown out of the tank drain daily. I can live with a
little compressor oil in the air when operating air tools, but it wreaks
havoc on my sandblasting (yes I do have a coalescing filter and drain it
regularly, but some oil/water is still getting by, especially when blasting
for any length of time).
The problem is, replacement parts from Sears are outrageously expensive (at
a minimum $75, and perhaps quite a bit more because it's not clear if their
ring "kits" include just one ring or a set).
Frankly, if I'm going to spend the money, I'd rather buy a large
vertical-tank stationary compressor, but I don't have the space for one now
(and with a baby on the way, it's out of the budget).
Any ideas on solving this problem without dumping a lot of money? Are
generic rings typically available for compressors (and if so, how do I find
the right ones)? Would honing the cylinder walls help? Would a better air
filter solve my problem?
Thanks,
Michael