Rebuilding FedCom warships before the Clan invasion

How could the Federated Commonwealth rebuild its warship fleet after the Fourth Succession War in preparation for the Clans?

It cannot simply rebuild the fleet: working WarShips are effectively absent outside ComStar, and the compact jump-drive technology needed to construct them has been lost.

The immediate answer is that the Federated Commonwealth cannot rebuild an operational fleet under the conditions posed. Surviving hulls outside ComStar are distinguished from working warships: some might remain as defensive platforms, but their jump drives no longer function. In the setting described here, an ordinary JumpShip's drive accounts for about 90 percent of its mass and allows a jump of up to 30 light years. Such ships have little propulsion for moving around a system, so attached DropShips make the journey to planets, taking roughly a week in the example. A WarShip's compact drive uses about half its mass, leaving room for propulsion that lets it manoeuvre after arrival. The manufacture of those drives and ships has become lost technology. Possessing a few old hulls therefore does not provide a fleet that can simply be expanded before the Clans arrive.