Jin did not build a Mecha Titan from scratch since he didn't have the resources nor the money to accomplish such a feat. Instead, he had used the Titan Knight from the Dungeon World and constructed the Mecha around it. (So much for not asking his minions for help, but he did prepare a nice present for Titan Knight, plus it was unaware about the feast.)
As the exterior was already in place, Jin mainly focused on repairing the internal parts as analysed by the System and gave him a new pair of 'legs' to boot.
For now, Jin used the caterpillar tracks from the Sherman Tank blueprints from Hou Fei. He would have preferred to wait until Amura Rei shared some of their world's Mecha blueprints as a reference, but as time was of the essence, he needed to use the Titan Knight now to catch the giant monsters for the grand feast.
