Bought a used kayak from a coworker, but he lost the drain plug. Would any plug fit, i.e. one off of amazon? Or should I replace the entire drain plug unit?

Thanks for the help!

Lots of ways to get by until you get the real thing, especially if drain is on top of stern deck. Jam a big wad ball of electrical or duct tape into it, or even bubble gum. If its just plugs for scupper holes you can usually find a universal plug that will work.