What should I do, as I disbursed a portion of my zakat to a man whom I thought was a Muslim, then I became doubtful of his Islam and could not find him?

One must ensure that the recipient of zakat is indeed eligible. If the zakat payer gives their zakat to someone they believed to be eligible, and then doubts their eligibility, such doubt after an act of worship does not affect it, and the default is that it is valid. However, if the zakat payer becomes certain that the recipient is not eligible, they are obligated to re-pay the zakat according to the majority of scholars, because zakat is a right for its legitimate beneficiaries. But if one pays zakat to someone they know or believe to be among its eligible recipients, and then doubts their eligibility, re-payment is not obligatory.