CEL was on, so read the codes using Icarscan.
0174 - Bank 2 to lean
0171 - Bank 1 to lean
1559 Power supply without engine operation

It was smoking at startup, but generally running fine. I decided to replace the air-oil separator. When I removed it, the old one was torn, so I thought I was on to something. Put the new one on, cleared the codes, and started it up. The CEL comes on instantly.
0174
0171
1559
and now
1081 - Air mass ahead of throttle valve, partial load
1094 - Air mass ahead of throttle valve, implausible

I chose to replace just the diaphragm and cap, not the entire thing. After getting codes, I sprayed some starting fluid around the cap, and no change in the engine speed or tone.

Where should I look next?

 

Smoke test the intake. Sounds like you're getting unmetered air into the motor from a leak or cracked hose.

Also, I've seen where those diaphragm kits don't actually fit properly but no pwrisnal experience with them.

 

Right, you were. The brittle, brittle plastic hose from the oil separator to the throttle body split.