Your (4) suggests taking parts from a broken C64. This
C64 therefore has
a fault. If that machine and the machine you're repairing have the same
fault (likely if there are small number of 'stock faults') or a fault in
the same chip, then you're not going to get very far...
How long does it take the DEC field service agent to fix a flat tire?
Depends on how many flat tires he brought with him.