Sunn Beta Lead 30k s curve pot
Figure 1. The reverb section with the S-curve potentiometers R212 and R213.

The main thing to note here is that the potentiometers are three-terminal and that the wiper is grounded. That means that the same effect as the original potentiometer can be obtained with a linear pot but the wiper position to achieve the effect won't be the same and the musically useful span might be only a few degrees rotation of the pot.

If you can detach one of the legs of the good pot and plot the resistance vs angle of rotation you may be able to figure out how pronounced the 'S' actually is. 

Beware of articles that suggest that you can make your own S-curve with a linear pot and a couple of resistors between the ends and the wiper. That will, I think, generate an inverse S-curve.



Figure 2. Various potentiometer curves. Source: sengpielaudio.com.