First, please enclose code snippets in triple backticks (```) in the future so people who want to help can copy-paste these into an input file. Otherwise, there’s some manual processing we need to do. Anyways.
“use this wavefunction to compute the energy” means different things in different contexts. Your question, as I understand it, is how to use an imported wavefunction’s orbitals as a guess for an SCF computation. (For DFT, this will get you the energy almost immediately. For correlated wavefunctions, it won’t.)
As for how you do that, add the following to the start of your script:
import shutil
wfnpath = wfn.get_scratch_filename(180) + '.npy'
shutil.copy(path_to_old_wavefunction, wfnpath)
That moves your wavefunction to the place Psi looks for it when it reads.
…This should really be documented and/or simplified, and I’ll make an issue about it.