How do I fix this poor little model plane (pic related)? The propeller and the spinner aren't perpendicular to the plane's body.
I have all the basic /diy/ tools by the way.
It may not need to be perpendicular to the longitudinal center line. Most planes have built in down and left thrust. This is to offset torque and the center of drag being higher than the thrust line respectively. Most high wing planes have down thrust. So don't assume it needs to be be fixed. Is that a factory install or did you install the motor?
It's not supposed to be. The motors make too much torque for the size of the plane. If there aren't canted properly it becomes uncontrollable or you need to use all your control authority just to maintain level flight.
Is the shaft bent or is the motor just mounted that way?
If the latter, the solution is simply to remove the motor and add washers, shims or at worst rework the firewall to take out the excessive downthrust.
If the former, buy a new motor or at least a new shaft. Don't mess around with trying to straighten a bent shaft, it's not worth the trouble.