Are you allowed to write a kitzur of what you learn on chol hamoed?
Yes, it is permitted to write on Chol HaMoed. This writing is necessary for remembering your learning, and it is therefore permitted.
It is also permitted to write out the learning in full.
Note that it is generally permitted to write on Chol HaMoed (according to the Rema, 545:1), excepting special scripts such as a Sefer Torah etc. – though the custom is to refrain from unnecessary writing, and the Mishnah Berurah (5) recommends writing with a “slant” when writing letters and the like.