Is it permissible to perform Umrah in regular clothes due to a chest deformity that prevents wearing the ihram garments, and will there be reward for the Sa'i (ritual circumambulation between Safa and Marwa) in that case?

A deformity in the leg is not an excuse for not abstaining from sewn garments during ihram, as it can be covered by the rida' (upper garment).

A muhrim (one in a state of ihram) is forbidden from wearing anything tailored to the body, and this is by scholarly consensus for males.

It was narrated that the Prophet, peace and blessings be upon him, said to Amr ibn Zurarah when he let his garments hang low, citing a defect in his legs as an excuse: "O Amr, indeed Allah has perfected everything He created. O Amr, indeed Allah does not love those who let their garments hang low."

If you enter ihram wearing your clothes and do not abstain from them, your Umrah is valid, but you are sinful, and a fidyah (expiation) is required of you: sacrificing a sheep to be distributed among the poor of Mecca, or fasting three days, or feeding six needy persons.