For preliminary troubleshooting, you might try a spray or two of a cooling vapor such as the following product:
https://www.electro-tech-online.com/custompdfs/2009/05/6403321_MSDS.pdf
This might assist in the location of a defective component that is caused by excessive heat build up .... excess current flow.
There are one or two other possibly defective components that would not become apparent by merely using a component cooling spray.
Sometimes electrolytic capacitors fail due to aging, drying up of the internal electrolyte paste, or possibly voltage spikes which exceed their limits.
One possible candidate that might be defective is capacitor C2, which is a 22 µF tantalum capacitor.
Tantalum capacitors can be easily damaged, more so than other types of electrolytic capacitors.
Capacitor C2 returns a feedback voltage signal, from the action of the Q1 photo-transistor, and consequently affects the switching speed of the Q1 transistor.
If your oscilloscope is working, it might be instructive to observe the voltage signal at the positive terminal of C2, adjacent to R13, as you apply power to the circuit, and the rotor spins up, and then decelerates, as a result of something that is defective.
If C2 is replaced with a new part, care should be taken to orient the + and - leads correctly when soldering the part into place, and to not allow excess voltage or sparks to contact the new capacitor prior to installation.
I see that C1 is a 1 µF tantalum capacitor. You might check this one also.
C5 ... 0.47 µF tantalum cap?
C3 ... 0.1 µF tantalum?