Did you replace the crank case seals ?
These saws run in very dusty & abrasive enviroments and eat seals for breakfast lunch & dinner.
Quick & not particularly reliable test.
Pop a little starter fluid into the plug hole and pull the cord
repeat down the carb throat.
If it fires better ( or only ) when the fluid is put in the plug hole, suspect the seals.
Second test.
Remove the plug and squirt some heavy oil down the carb throat, pull it through till the plug hole no longer blows oil.
Pop the plug back in.
Engine fires again suspect the seals
The best thing of course is to do a vaccuum & pressure test on the crank but very few people have a vaccuum gauge & pump.
Lots of do dads now days but the same principles apply now as they did then.
If you bought the magneto units from a cheap on line vendor then they are to be considered suspect.
A lot of cheap stuff on line is the parts made for Stihl that Stihl's QC has rejected as not being of servicable quality so the makers on sell them to the scrap dealers who then resell to the distributors who sell to the on line vendors who sell to you.
Genuine OEM parts does not mean "Genuine good quality OEM parts".
When you are running a plant on less than 20% mark up you can not afford to toss out anything.