Is there a way to reseal a leaking intake manifold?

I have to agree with post #6. I think that you have a crankcase pressurization problem, with an oil leak as the symptom.
I would fix the symptom and then address the pressure problem. I used one way valves through the header collectors .
If you don't already have this, I would strongly recommend having an ignition switch and a separate starter button. Always have your engine turning over when you turn the ignition on. If you have a backfire on start up, it's better to have turning. That little pop off valve in the manifold is kinda for looks.