If it did not smoke before as you say, and there is no oil in the inlet tract, then that only leaves a few places.
I think your suspicions about th head arew wrong or certainly should be, if it is a decent machine shop ?.
I know it does sound like the valve stems/seals, bnut as they were not worn before can't see how they could be now ?
I would change the oil first and for a differnt brand such as SILKOLENE 1PRO S 15/50.
What torque setting did you use for the head ?
I know when my machine shop does heads they use a marker on the valves and then bung them in a container.
It is not beyond the realms of possibility that they have forgotten the stem seals, but if it had leaky valves before(most do) then it will develop more vacuum at overun and be much more likley to drag oil now than before into chamber, FROM ANYWHERE it can suck on.
Is it std what breather are you using ?
tabetha