Kalitta does quite a bit that doesn't have Boeing support. That is, where Boeing won't provide the technical support, Kalitta employs engineers and designated engineering representatives and designated airworthiness representatives to handle it directly, based on FAA approvals.
The classic is far from being economically non-viable.
Kalitta maintains a large store of spare airframes and powerplants, as well as maintaining a substantial shop. The company also handles outside work for competitors and other vendors.