Simple fuse box fix?

One thing to keep in mind... lets imagine every copper stand in a battery cable was broken except 1. Your test lamp would show power yet there wouldn't be enough current to operate anything. Since you said engine replaced recently, double check all positive and ground connections from battery onward, especially if you have those add on cable terminals at the battery. Just a loose bolt where ground cable attaches to block can cause the symptom you have. Good luck with it.