I want to modify the rate limit for Crons Monitors to allow more frequent check-in events.

Crons imposes a rate limit on check-ins to prevent abuse and resource overuse. The rate limit is 6 check in events per minute per monitor environment. The rate limit can not be increased. You may want to look at manually instrumenting the check-in for that monitor in a way that it is not sending check in events everytime it runs.