If you're happy to go with the flow rather than having a pre-conceived design, old kits are a fabulous source of raw materials.

I've stripped an F-104 Starfighter kit of it's wings, tail and landing gear, used Milliput to flatten out the cockpit, glued an assortment of bits on and made a rather pretty space cruiser (it's still in the loft somewhere...), and I once stripped a C-130 Hercules down, making a dreadnaught from the heavily disguised fuselage, and turning the engine pods into smaller support vessels. Assorted missiles have been turned into light cruisers, and I've still got a bits box of random old kit parts that periodically come in useful 😁