What would be acceptable compression on a 1.6 16V engine.. I'm ripping one out of a Tracker for my conversion and decided to pull the plugs and spin test for a quick compression check.. 3 cylinders were 190 psi and one was 150 psi. Would you consider this "Good Enough" to proceed with the swap without doing anything to the engine?

Or, should I consider having my head worked or a possible re-ring before the conversion?

I would consider a re-ring job due to the big diference between cylinders , but you could also take time to chek valve lash , I'am not certain if 16 valve heads use normal lifters or hydraulic ones. as a refernce my kick 8 valve carburated , had 160 psi some time ago. According to my service manual for the 8 valve the
standard compression for my model is 199,0 psi (yea right..)
service limit compression : 170 psi
and maximun difference between any two cylindeers: 14.2 psi
greetings

Would you consider this "Good Enough" to proceed with the swap without doing anything to the engine?

No

Or, should I consider having my head worked or a possible re-ring before the conversion?

Like ChileKick said, first of all check the valve lash. You'd kick yourself if you ripped the head off and it was just a valve that had closed up. However it could be more serious, a head gasket problem or a burnt valve. Cracked heads and blocks aren't unknown either so it needs to be investigated.