What is the ruling on praying in clothes that are likely smuggled (stolen), and must the prayer be repeated if it was performed in them forgetfully?

Wearing these clothes is permissible according to Sharia, and the basic ruling regarding them is permissibility as long as they are owned by the one who sells them. The fact that they are smuggled does not necessitate that wearing them is impermissible. The seller, however, incurs sin if they do not pay the due state taxes. The fundamental principle in these matters is permissibility, so wear from them whatever you wish, and your prayer in them is valid.