How to remove water-based paint?
I suggest to remove the stain with water, e.g. rinsing the part of the cloth or using a steam cleaner which has the advantage to be able to clean smaller parts of the cloth. Afterwards the gelatin will have (at least partially) resolved at the respective part of the painting (along with most of the watercolor). 

To retain the structure that is needed to paint on the cloth, gelatin has to be added back on to the parts where the stain has been removed. For this the entire cloth has to be damp (not wet) before the gelatin is added to ensure flexibility of the material and to allow it to stretch evenly along the frame once dry.


remove stain by rinsing with water or with a steam cleaner
cautiously dampen the cloth from the side of the cloth you have not drawn on (this will minimize bleed of the colors). This is the most delicate part. The cloth only needs to be so damp that it will have some slack on the frame. 
'paint' the part of the cloth where the stain was removed with gelatin
let dry


At this point you can repaint the cloth.