gunked up heater ducts - how to "clean"?

[Flojo]

I guess many of you now that phenomena: you overhauled you car. You got yourself new heat exchangers... but when opening the heating it still smells of hot burnt oil.
I guess due to the gunk blown into the heat tubes due to years of the PO keeping that leaking engine dripping onto the old heatexchangers.

Any chance to solve this (without cutting the rockers open and retreiving the old heat ducts)?

I guess no(?)...