Is there an issue with a mohel performing a bris milah a chalel (the child of a male cohen who married a jewish divorcee)?

Shalom, Thank you for your question. The obligation for brit milah applies to every male Jew, irrespective of their lineage. This being so, a chalel certainly is obligated in the mitzvah, and the mohel will preform the brit as on any other Jewish baby. Blessings.