Before you worry too much about blowby, it may be worn rings, but it may also be stuck rings from carbon buildup. I'd get a good diesel carbon remover or Sea Foam and load it up. I have a '63 Ford 2000D that was smoking something awful and after half a dozen cans of SF you have to look hard to see it smoke...unless you yank the throttle from idle to WOT.
I didn't follow the instructions on the can of 1 oz per gallon. I'd put like 2 full cans to a tank of fuel. I've recovered numerous small engines that I thought were goners, both 2 and 4 cycle with it. It's made for all internal combustion engines....read the label.....been around for 50 years.