Alrighty, update time.
New nozzle check valve did nothing, saw still ran exactly the same - would only behave somewhat normally with the mixture screws seated. Loosen the screws even slightly and it would choke up and die.
New Proline carb had a sticky choke, but adjusting the choke plate fixed that.
Fired up, adjusted properly, ran and cut great.
Whatever two stroke oil the previous owner was running makes big blue clouds of smoke and smells terrible. Piston and cylinder look great, so they did something right, but I'm looking forward to burning all that out.
So far so good.